The factory was usually pretty good about using the same number for the same part; as it was cheaper that way. If the factory part numbers are different, then the parts are usually different in some way. Of course, sometimes a new number could replace an old one; for example the later non-sleeved thermostat is listed as a replacement for the earlier sleeved thermostat. But if they knew up front the parts were the same, they got the same part number.
Sorry I can't be any more help. But I've compared the Stag parts to the TR6, and practically everything is different. Tie rods are thicker, rack housing is bigger, and so on. Looking at Rhodyspit's photo, I suspect you'll find the same thing between TR4 and Spitfire. ISTR the TR4 rack looking rather beefier than that.
The Moss website is another place you can check, to see if parts are the same. Just like the factory, they would rather stock one part instead of two, so they use common part numbers when they can. Looks like the boots and pinion are the same, but not tie rod ends. You'll have to check the others for yourself