I bought this rear off the internet for a 70 W-30 restoration project Never took it out of the crate only checked the TM axle code. After 3 years I was prepping to install and noticed the casting number on the center section didn't
seem right nor the date code. Could anyone shed light on the de-coding for me.

 

It’s definitely not a 1970 O-Type.

That’s the same casting number as my 1968 TL code O-Type, which is a 3.42 open HD with 31 spline axle shafts.

 

From Supercars Unlimited page:"There are three basic 12 bolt "O" Axle housings. The 1967 through 1969 housing was used for all models except wagons. The original part number for this housing is 230727 and will have the casting number 39413 or 394139. This same housing was also used for the 1967-1968 models with 3.42 through 5.00 gears, but because these models used larger carrier bearings, the housings were machined accordingly and therefore required a different part number. The part number for the 1967-1968 3.42-5.00 housings is 320575.

In 1970, due to the axle bearing design change, the housing part number 231000 was used for all models except wagons and of course the W-27 aluminum housing. The W-27 aluminum housing used the part number 231042. The casting number for the non W-27 housing is 402227. The casting number for the W-27 housing is 406784."

https://www.supercarsunlimited.com/i...id/diff_id.htm

The first picture (screw)334(screw) is the Julian date of casting for the particular model year. So 334th day of 1968 or 1969 .

 

Unfortunately, that info from Supercars is incomplete. There are more than three basic 8.5" Type O housings for the A-body cars, and their list completely ignores the 8.5" Type O housings used under the small block B-body cars. The Type O axle was released in May 1966 and was originally used under 1966 models. The 1966 housings have different spring perches from the 1967-up housings and thus have a different part number. I have not been able to find that P/N, unfortunately, but I suspect it used the same center section casting as the 67-69 axles. Also contrary to what Supercars says, the flattop wagons used the same P/N housings as the same year coupes and sedans. Only the Vista got a unique housing due to the larger outer axle shaft bearings. Again, the center section casting number would be the same as for the coupe/sedan axles.

For completeness, 8.5" Type O housing part numbers (not casting numbers) are as follows:

1966 A-body - ???
1967 A-body - 230027
1967 Vista - 230028
1967 B-body - 230029
1967-68 A-body perf axle - 230575
1968 A-body - 230383
1968 Vista - 230384
1968 B-body - 230385
1969 A-body - 230727
1969 Vista - 230728
1969-70 B-body - 230729
1970 A-body - 231000
1970 Vista - 231001 (note that this P/N superseded 230728 for the 1969 cars)
1970 W27 - 231042

Count 'em. That's 14 different 8.5" Type O housings.