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That said.....

I may be wrong. The answer above is correct for the 1970 model year, but I checked some 1969 model year broadcast sheets I have here & some have numbers there & some do not.....

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I did a quick look through my sheets...

In a copy of a GG BS decode I have, GG does list this as being a time code just as Barry explained.

It looks like Hamtramck started using the time code in March. It doesn't appear to show up on STL sheets until the first week of June.

LR 246139 (324 SPD)does not have the time but the time does show up on 259416 (416 SPD). That's as close as I can get for LR. So it looks like LR started about the same time as Hamtramck and about the same time the A12s were built but it is not an A12 specific code.

I have a C body with the code on a 507 spd BS.

So it looks like the time code was introduced about late March or April but doesn't show up on BS at all plants until June.