Even though this is a TR4 gearbox, maybe the photos will help...
In the above photo you see the location of the serial number arrowed. It should be in the same place on your car. It's stamped into the metal, not embossed as a part of the casting.
This particular photo doesn't help much with your questions, but it points out a few other variations as compared to TR2/3 gearboxes. (Note: one exception, TR3"B" would have same gearbox as TR4.)
Notice the bellhousing mounting flange in either photo. This is how thin it was on TR3 and TR4. It was beefed up once or twice later on.
Also notice how this gearbox has to stand on a couple blocks of wood, or else the input shaft (which protrudes approx. 1/2") would hit the floor and cause the gearbox to tilt to one side or the other.
Looking at your photo, I can only guess it's a TR4A, TR250 or early TR6 gearbox. But, I was under the impression the heavier mounting flange (which you have) wasn't used until the 6-cyl. cars.
If it has a "CT" prefix, it is TR4A.
If it has a "CC" prefix, it's TR250/5 through 1972 TR6. CC20000 or higher indicates a 1970, '71 or '72 build date (TR6, of course).
If it's "CD", the gearbox is from '73 through '74 TR6 (this series started with CD75001 and went up from there).
If "CF" it's TR6 from 1975 or '76.
The embossed numbers in your photo are the *part number* for that particular piece. They are cast along with the part and are not sequential like a serial number. That's the part number of the tail piece, in particular. "Standard" is the name of the manufacturer: as in "Standard Triumph". Throughout the car, some parts continued to have either the "Standard" or "Stanpart" label on them even after British Leyland took over in the early '60s.
Regarding the "anti-rattle" plunger and spring you ordered and installed, that part is pretty much universal to all the gearboxes (and just as frequently lost when the shift lever is removed). So I'm not surprised it fit fine, no matter what year was used for reference. I think at some point they used a ball bearing, but the replacement part is now the same for all TRs AFAIK.
As a side note, the gearbox now in my TR4 has an "MG" prefix on the serial number, meaning it came out of a Triumph 2000 (so it would date from 1963-69). This doesn't really tell me, though, if the internal parts are the slightly later ones as well.
For the large part, as complete units the gearboxes are interchangeable, with their most notable external differences being the length of the input shaft and the thickness of the mounting flange (TR2/3 g'boxes have more differences). However, there are enough differences internally that it's useful to know the specific model, if, for example, a rebuild is ever needed.
Glad to hear you have the 3/4" wide shaft bushings in the bellhousing. Those are definitely preferable over the later narrow type. However, although you have the earlier type of bushings, it looks from your photos like you have the later type of cross-shaft that doesn't provide any means of greasing them (you can see the 90 degree grease zerks in the ends of the shaft on either side, in the above photos).
I hope this clarifies.
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