The engine code would suggest you have one of the last TR4 engines (stopped at CT40304), for which I believe the Stombergs are correct, and would also explain all the performance choking emissions equipment. Apart from breathing and emissions, the engines are nearly identical, so someone at some point probably just swapped the tired TR3B engine out one afternoon for something with a little more pep.
If you're not worried about the numbers matching (as few triumph owners seem to be since they never even matched from the factory!), the good news is that you can return the rest of the goodies back to stock by picking up a set of H6 SU's, appropriate intake and exhaust manifold, valve cover, and all that emissions junk that chokes off power, and have yourself a completely original looking TR3B. If you just want a good running car (though arguably, SU's are more reliable), leave it all on. You should be able to find Stromberg aircleaners for TR4 or 4A's that will work just fine.
Incidentally, I have a TR3A block with all the goodies still attached if you're interested in rebuilding used parts to bring your engine back to stock. Let me know.