A “numbers-matching” claim rests on three separate records that answer three different questions. The casting number says what family of block the foundry poured. The stamped suffix says what the engine was assembled as. The VIN derivative says which car it belongs to. Nothing is uploaded; decoding runs entirely on your device.
Where to look. The pad is a machined flat on the block, forward of the passenger-side head on a V8, near the distributor on an inline-six. The stamping reads fmmddaa: f is the engine plant, mm the month, dd the day, and aa (occasionally three characters) the application suffix. The casting number is cast into the block, not stamped: right side on a six, rear-left near the bellhousing flange on a V8.
1967–1981. The 1967–1969 tables are the Camaro Research Group's; 1970–1981 are GM's engine identification pages, with codes that appear only in the Camaro White Book flagged. A few 1970–1980 codes are hard to read on GM's pages and are marked as such. 1967–1969 stampings follow the CRG key (type letter, model-year digit, month letter, day, suffix). 1970–1980 stampings are read from GM's own pages: a type designation, a source letter, the model-year digit, a month letter and the day. The 1981–1992 assembly codes are Chevrolet-wide service tables that map to RPO numbers rather than to Camaro transmissions, so the decoder does not read them. The Camaro Research Group's 1967–1969 table (every two-letter code, with its ratio, housing and Positraction), then GM's kit pages: four to six ratio codes a year for 1974–1980 and fuller tables for 1982–1992. GM's 1969–1973 kit pages print no axle codes. A code is only ever read against its own year.
Read the assembly stamp in four parts
The suffix must be read for the car's model year. Some suffixes have three characters. This diagram shows the format, not a particular car's stamping. Original CamaroFacts diagram. Camaro Research Group reference.
The same two letters, a different engine
Suffix codes were reused. MO is a 302 Z/28 in 1967 and 1968, and a 427 COPO in 1969. MR is a 375 hp L78 in 1967 and a 350 hp L34 in 1968. Read against the wrong year, a code turns an ordinary car into a rare one on paper, a mistake that turns up in sale listings.
This decoder will not do that. A suffix is only ever read against its own model year, and a code that is not in that year's table is reported as absent rather than borrowed from a neighboring year.
Three records, three questions
Compare all three with the rest of the car. Correct-looking numbers alone do not establish authenticity. Original CamaroFacts diagram. Camaro Research Group reference.
None of this settles authenticity on its own. Cross-check the pad against the cowl tag and the VIN, and read the full identification workflow before acting on any of it.
Did you know
Axle codes get reused as surely as engine suffixes. GM's 1967 kit lists PW as a 2.73 axle; its 1975 and 1976 kits list PW as 3.08; and the 1974 kit uses different codes altogether (CA 2.73, GY 3.08, CM 3.42, CG 3.73). Two letters alone can't give you a ratio until you know the model year.
Transmission date stamps use a month alphabet that skips letters: A Jan, B Feb, C Mar, D Apr, E May, H Jun, K Jul, M Aug, P Sep, R Oct, S Nov, T Dec. GM's 1969-80 kits print that chart and CRG's drivetrain guide matches it, so P9T11B reads December 11, 1968. GM's 1967 kit stamps the month as a number instead ('5 denotes May').
Two month alphabets appear on GM parts, and they differ. Cast-in dates on blocks and heads run A-L with I as September; stamped dates on components skip I, so J is September and M is December. A cast 'B129' is February 12, 1969; a stamped '8C26' is March 26, 1968. CRG also says an M in a block's cast code is a foundry test code, not a date.
CRG gives the chronology a correct set of stampings must follow: the block casting date precedes the engine assembly date, which precedes the vehicle assembly date, and the engine date almost always precedes the cowl tag date. Most engine dates fall less than 30 days before the vehicle build, some 30-90 days before, and more than 90 is an odd exception.
Late-1969 SS396s can carry a 1970-style three-letter engine code (the 1969 JF became CJF). CRG cites the September-October 1969 Chevrolet Service News: those big-blocks are actually 402 cubic inches, a .030 overbore of the 396 that was still advertised as a 396. GM's 1970 kit lists the engine as '402 (SS 396)'.
CRG says that in 1967 generally only Z28 and SS engines carried a partial VIN, and every engine was stamped from 1968. GM's Protect-O-Plate explanation in the 1967 kit says assembly plants stamp the plant letter and serial on all 327, 350, 396, 409 and 427 engines except Corvette's, and it doesn't name the 302. The two accounts differ.
By 2001 GM documents the check explicitly: every engine and transmission is stamped or laser-etched with a nine-character VIN derivative (division, model year, plant, six-digit sequence), matched to VIN positions 3, 10, 11 and 12-17 to see whether the original unit is in the car. The 4L60-E's own stamp adds a Julian date and a shift letter: A, B or J is first shift; C, H or W second.
Casting-number cautions from CRG. The 3932388 327 block is sometimes called rare but was commonly used in mid-1969. The 3959512 small block, used mostly for warranty and service, is easy to confuse with 3963512, the 427. And 3970010 appears only in the second half of 1969 production, so a 1967 or 1968 car carrying it has a later block. All 1967 small blocks except the SS350 had 2.30-inch mains; the SS350 and 1968-up had 2.45.