I believe you're describing the plug at the bottom of the early CDSE carburetor (a CDSE 175, if from a TR6).
It's a big, heavy, cast brass plub on the bottom, protruding through the float bowl. It has a large slot in it, as if for an enormous screwdriver. It unscrews counterclockwise, just as if it *were* a big bolt.
There is no screwdriver for equipment with slots that size; it's expected that you'll use a piece of steel bar the same width as the slot, stick it in the slot, & turn the bar. For the bar, think of the metal rule part of a tri-square (although I think those are actually a little too thick for the ones on the brass bowl plug).
The problem here is that age & old gas can glue the threads together tighter than you'd at first imagine. Probably it is so tightly glued now that attempting to unscrew it with a bar in the slot will likely just break off the shoulders of the slot in the brass plug.
If the screws from the float bowl are removed, the bottom of the float bowl *should* just come straight down over the plug (the seal to the float bowl is an o-ring, the threaded part that's stuck is the bottom of the carburetor body).
However, when things are stuck to this degree, getting the float bowl apart is usually also a problem. With the brass plug in the hole in the float bowl & anchored to the carb body inside, you have no "wiggle room" that will likely be needed to get the bowl off.
This is very likely why the design was changed, & the only change here was to use the short snap-in plug to replace the big brass screw-in plug. It is a direct replacement. The threaded brass plug arrangement was probably their first idea to close the hole in the bottom of the float bowl, as the hole in the bottom of the float bowl & threaded section in the carb body were pre-existing parts of the CD carburetor design before the CDSE.
At any rate, the important thing to remember is that if you damage anything, it should only be the brass plug. Beating the float bowl around or otherwise affecting the mating sufaces at the float bowl or any other surface on the carb body can result in an unusable carb.
The place to start is holding the carb body. You need a vise (if you don't have one, you will need it for many other things, & they're cheap at Harbor Freight or whatever, & will be the only tool you'll need to get this plug out). Youl'll need to create a sort of "soft vise".
Strip the carb body so that you have the bare faces of the flat flanges that mate with the air cleaner & intake manifold clear. Open the vise up, get a couple of short pieces of wood like 1x4, & place the wood pieces next to the jaws. Put the carburetor upside down into the vise (carefully, don't bang & nick things), between the pieces of wood. In other words, there's a piece of wood between the carb body & the vise on each side, so that the metal of the vise doesn't touch the carb body. Close the vise just enough to firmly hold the carb.
It is possible that by heating the brass plug with a small torch like those little handheld butane torches that the bond gluing the threads on the plug will break & that it can be unscrewed with a bar in the plug's slot. It is just as likely that heat won't help, or that the heat will transfer to the aluminum & cause problems like warping of some surface on the carb body.
If you can unscrew it with a bar in the slot, congratulations. If not, you will have to mar the brass plug's sides by getting a pair of vise-grip pliers around the brass plug head & just turning that thing. It will unscrew & come out, but the edges of the brass plug will retain a sort of knurl from the teeth of the vise-grip pliers. It may help to give the brass plug a light-ish tap or 2 straight down with a piece of wood between the hammer & the plug.
The threads in the other end of the plug are about 1/2" long. The length of the entire brass plug is about 1 3/4".
After the plug is unscrewed & removed, the knurls that the vise-grip pliers put on the brass plug can be made less sharp & cleaned up by running a wire wheel over it. But it won't look smooth again, more like the outside edge of a quarter.
If somehow the aluminum threads in the carb body which mate with the brass plug are damaged, don't worry about it. They are left over from the CD carb's adjustable jet & don't come into play in the CDSE. However, you certainly don't want to break that part of the aluminum carb body which has the threads in it, so go slow, heat & tap cautiously, use firm, easy, steady pressure, stop if worried & tap & heat a bit again.
Since it is a problem for rebuilding, do what Stromberg did & do not put the brass plug back afterwards. Simply snap in one of the float bowl plugs as fitted to the later carbs.
As this seems to be a CDSE, I'm assuming the 175 is similar to the 150 used on the GT6 & has an assembly called a "starter box" that is used for the choke. Examine the disc with the graduated holes in the starter box. These discs are deteriorating with age, & if it's pitted, etc., badly enough, gas will go by even when the choke is off & the car will run rich so badly that it can't be leaned out enough (fouling the plugs at idle).
My friend Pierre has had success with some of these badly corroded discs by polishing them flat again with fine emery paper (be aware that it doesn't all come out exactly as it was before, as the graduated holes have little countersinks around them which change their dimension when some surface is polished away).
At any rate, check those discs. They are NLS from most Triumph suppliers, & are an essential part. It would seem with the problem arising there that there would be aftermarket replacements made, but perhaps nobody's been asking for them - just chucking the Strombergs for Webers if the Stromberg's can't be made to work properly, or worse yet, chucking the whole car.
Good luck.
Short answer: carefully secure the carb body in a soft vise, clamp some vise-grip pliers on the brass plug, & unscrew it straight out. Replace threaded brass plug with commonly-available snap-in plug.
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