1970 frame decoding

Hi guys

I'm looking for some info on the decoding of A.O. Smith frames. The previous owner of my 70 SS gave me a spare frame(has a 10 bolt rear) he had from a parts car and curiosity got me and I've been trying to decode the stampings. I haven't found a VIN yet but tomorrow I'll look some more. Heres what I did find stamped on the rear drivers side frame rail:

3960733 S(with a circle around it) 278
3 17 S 1 XU

From reading the archives and posts from Dale M, Dave Birdwell, and others I have gotten most of the info. 3960733 is the factory part number for a 1970 2 door sport coupe frame. The S(with a circle around it) is the frame manufacturers I.D. and decodes to the A.O. Smith company. But the 278 seems to be a mystery. Dale had hypothesized in some older posts it was the day of the year the frame was built but research on other frames(see below) seems to contradict this theory. Has anything new been discovered to explain this number?

The 2nd line would indicate a March 17(3-17) build date. But the S is suppose to represent the year according to GM parts manuals. All the A.O. Smith 3960733 frames I've seen don't have a number but a letter instead. Is it possible that A.O. Smith used a letter to denote the year instead? One member here stated years ago that the "S" was for 1969. He didn't say where he got the info from. Next is the 1 which I believe is for the 1st shift. Lastly the XU code, which allegedly was used on the build sheet to identify the frame and is supposedly one of the codes to denote a 2-dr sport coupe frame. Any info that can fill in the blanks would be appreciated.

I have included as many A.O. Smith frame stampings as I could find in the archives to possible help decipher the pattern A.O. Smith used. I included both 3960733 and 3943331 part numbers as both seem to represent 1970 2 door sport coupe frames. Interestingly, I noticed all the 3943331 frames have 191, 192, and 193 numbers and XK's. The 3960733 frames have 271, 275, 276, and 278 and XU's. One 3943331 frame says XX but I believe the owner misread the XK and thought it was XX.


3960733 S 276
1 8 N XU

3943331 S 192
1 21 S 2 XX

3960733 S 276
1 21 N 1 XU

3943331 S
2 27 S 2 XK

3960733 S 278
3 17 S 1 XU

3943331 S 193
3 27 S XK

3943331 S 193
4 24 N 1 XK

3943331 S 193
6 17 S XK

3960733 S 271
9 30 N 1 XU

3960733 S 275
11 18 S 2 XU

3943331 S 191
12 9 C 1 XK

3960733 S
12 18 S 1 XU

3943331 S 191
12 23 S 2 XK