Welcome to the board Gary!
L-48 was made from 1967-1980. The L-48 is the original 350, available only in the Camaro or Chevy II/Nova in '67 & '68. In '69 it was used in almost every thing; Camaros, Corvettes, Impalas, Chevelles & Novas. From '75-'80 it was available only in the Corvette.
L-48's use a hydraulic cam, 4bbl Quadrajet, use cast pistons, two bolt main caps, "Pink" Rods, #0014 Blocks & #993 heads. Power output ranges from a high of 300HP(gross in the Corvette) down to 175HP(net).
The L48 was the standard engine in the 1971 Chevrolet Corvette. It produced 270 hp and 360 ft-lbs with an 8.5:1 compression ratio.
The 1976-1979 L48 was the standard Corvette engine and produced 180 hp and 270 ft-lbs. The 1980 L48 had at best 190 hp and 280 ft-lbs from only an 8.2:1 compression.
In 1973 the "L-48" had cold air induction (throttle activated) and developed 190 hp (net). Beginning in 1974 the hp was reduced for several years until it reached a low of 165 (net) in 1975, before rising again.
The L46 was an optional engine on the 1969-1970 Chevrolet Corvette. It was a 350 hp (380 ft-lbs) version of the L-48 with higher 11:1 compression that still used a hydraulic flat tappet cam.
If you want to know if you have a 12 bolt bend down and count them. The 10 or 12 bolt designation refers to the number of bolts securing the rear inspection cover onto the housing. If you have a Chevy 12 bolt it will have a 12 bolt cover. As to the gears crawl under the rear and look on the passenger side of the housing for the numbers stamped into the axle tube. They will tell you everything once decoded (we can look them up for you). It is probably an open two series gear (most common) with a small probability of it being a low three series gear.