The shortblock in your Vette was assembled in Flint, Michigan (V); on January, 21st (0121); and was originally designated as a 1973 -74 350 engine (vin code T, L82) with a manual 4-speed transmission (all this means "CLR").
Now, this code was typically used to identify the engine regular production order (RPO) number, but it basically it is also referred as the Short-block code.
What you should see in front (or meaning before) that code the last 8 (I believe) digits of the Vette's vin. Can you see this?
Also, CLR was also used for the 1978 (Vin code L, L48) for Vettes and Monte Carlos', so that is why the vin portion before the short block code is important in deciphering your Vette's originality.
p.s. You should be able to find in your local book store (or eBay) a "Chevrolet V-8 Small Block ID Guide" or something like that. There are also Corvette Parts Interchange" manuals available, but mine does not have a dedicated section to decipher every Chevy V8 code. It just tells you whether a particular code was also used on another car line (i.e. Z28 Camaro, Impala, and so on). I have a pocket-size one from MSA-1 that is perfect for car shows or take along with you at auctions or when looking a Vette for sale by a private individual.
