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Yes, the frame part numbers were different for 1971 and 1972. However, the frames are interchangeable. The 1971 frame for 4 speed cars was GM #3987882 and for THM-400 cars it was GM #3987883.
The 1972 frame for 4 speed cars was GM #6263620. For 4 speed cars it was GM #6263623.
According to Joseoh Tripoli's book, Corvette Chassis Restoration Guide, the frame stencils for 71 and 72 are not the same.
There is a list of frame stencil numbers for the different years, but I have an old copy and I know there were some errors in this edition. I recommend you try to lay your hands on a newer copy.
From his book, there are three unique stencils for 71 frames, and four unique stencils for 72 frames. The frame part numbers given are esactly the same as those given by Joe Lucia above, but there is also a hyphen and a three numeral suffix. I would say if your stencil is no longer visible, it will be tough to come up with the original stencil numbers unless you know when in the model year each of the suffixes were used.
Thanks, Joe. Your are da man. I believe you meant to say THM cars for #6263623.
My July 12 (21,467) build '71 automatic has the '72 part no. stencil, and also has "1972" in grease pencil. An inventory mix-up as model years transitioned, maybe? Also, it has two stencils (side rail and behind front wheel), one with 6-7-71 date, and one with 6-8-71.
At the beginning or end of model year, "carryover" is possible for many parts. Not ALL parts, but many parts. In this case, the frames would be among the parts that could easily be utilized for either model year. The 1971 and 1972 frames are fully compatible and interchangeable. I'm sure that there is some difference(s), but I'm quite sure that they're minor and don't affect compatibility.
Tripoli shows the 72 frame stencils as having the 140 and 141 suffix for an automatic transmission car. Early frames were probably stenciled 140 with the suffix changing to 141 during the model year. In 71, he shows automatic transmission cars with suffixes 120 and 121.
For 70 manual transmission car frames, Tripoli's shows FOUR suffixes being used through the year...80, 81, 82, and 83. However, his information was incomplete in my case...My car was very late 70, and the frame was stenciled 3974600-83X; his list only shows the numerical characters, and not the X
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