Glynn,
There are listing for Suzuki frames and motors vs 'model year' (ie L, M, A, B etc)
For the 380 the
starting numbers for each model are given as this:
GT380 371cc 2-cycle triple [more]
1972 GT380J
- Frame: GT380-10001
- Engine: GT380-10001 [more]
1973 GT380K
- Frame: GT380-30919
- Engine: GT380-32894 [more]
1974 GT380L
- Frame: GT380-48732
- Engine: GT380-53849 [more]
1975 GT380M
- Frame: GT380-65984
- Engine: GT380-73229 [more]
1976 GT380A
- Frame: GT380-86754
- Engine: GT380-97353 [more]
1977 GT380B
- Frame: GT380-95063
- Engine: GT380-108577
What this looks to mean though is that your current engine is a 380M as is the one you're offered. I don't recall the changes but at least you'll know it'll fit

LMAB all use the same en bloc carbs but I think there were porting changes that didn't make a huge difference and they look pretty much identical.
Cheers,
Mike