Buick Rear End Identification Decoder Ring Help?

[ChrisSD]

I have searched the internet high and low for the decoding of this rear end. The seller told me it was out of a 68 GTO which I believe is not true, but the only codes I can find on it- other than the casting # 1375689- is on the axle tube and it is "LA 42D". I have attached some photos (sorry about the quality) which show the #'s and the diff style. My thought was that it may be from a '66 or earlier Buick rear end series 42 with a 2.78 axle. Am I warm?? Thanks.