Doug's:

part #2881489 ?? 3.23s (42-13) same as cast number on carrier??
1-7/8" diameter pinion stem (10-spline)

ring gear 42-teeth stamped #2800270
pinion 13-teeth stamped #2800271

listing shows this Ring & Pinion for use in the following carriers:
#2881488 ('69 & up)
#2881489 ('69 & up)

AND/OR

part #3744485 3.23s (42-13)
1-7/8" diameter pinion stem (29-spline)

ring gear 42-teeth stamped #2800270
pinion 13-teeth stamped #3507396

listing shows this Ring & Pinion for use in the following carriers:
#2881488 ('70 & up)
#2881489 ('70 & up)

I've found some conflicting additional info - is the above correct?