They are, just depends how much work you want to put in.
Often you would have to go back to original Meriden records - engine build books, bike build books, dispatch books. Aiui, TOMCC has copies, originals are in the VMCC (Vintage Motor Cycle Club) library.
Reason you need all three (or two from the introduction of the date-code-5-figure-number format) is a given bike was not always recorded the same ...
Triumph T120C, T120TT and Trophy Specials gives an idea of the identification work involved in compiling accurate-ish numbers.
More detail is available on Justin Harvey-James
Triumph C-Range website and in John Nelson's
Triumph Tiger 100 And Daytona, Harry Woolridge's
The Triumph Trophy Bible and Lindsay Brooke's/David Gaylin's
Triumph Motorcycles In America:-
. TMIA says all of H65573-H67330 went to the US.
TTTB says H66124-H66976 were T100C so H65573-H66123 and H66977-H67730 were almost certainly T100R.
. Justin was a volunteer in the VMCC library when he was building his website:-
... interesting to note are:-
.. 30 3TA engines from PC[0]5516 - these must have been for an existing fleet customer as 3TA (and 5TA) were not available to the general public after the end of 66;
..
TT100AD confirms XC06279 was a Daytona, but does not specify T100R or T100T;
..
TTTB says the first T100C with a date-code-5-figure number was XC07583;
... are AC09937-AC09965 (although that is 29 numbers) aiui for the Garda (Republic Of Ireland police);
... AC09966-AC09987 went to the "White Helmets", the display team of the-then British Army Royal Signals regiment (the Royal Signals having historically used motorcycle dispatch riders);
.. I was surprised how 'late' in the model year Meriden was still making US versions; while
TTTB says the last T100C was EC19426,
TMIA says the last 69 500 that went to the US was HC24527 = July 1969.
A final curious anomaly is, while the website says the last 69 500 was JC24839 and John Nelson's
Bonnie says the first 70 T120(R?) was JD24849,
TTTB says the first 70 TR6 was HD23795 ... Could have been a foul up - whoever stamped engine numbers spent most of January 1969 stamping "AD" ...
You would need to do that if you wanted to know, say, T100T and T100S in 1969, no books about them ...
Not as fast as the mid 1960's, when there could be separate East Coast and West Coast variants of a US version ... BSA Group MD Harry Sturgeon wanted big increases in annual production numbers from 66, it might or might not be coincidence the number of different versions/variants actually produced at Meriden began to decline from around the same time?