I have a 400hp a/c vert. built Oct. 2, 1968. It is my understanding that it could only be ordered with a 3:08 rear end. What would the housing letters be and what would it be dated? There is an "AL", "AT","AW" listed in " Corvette By The Numbers". Anybody have what I need?

 If you go to Barnes and Noble book store,They have a book called Chevy I D Numbers and codes, You can look it up there,My corvette black book does not show it,,rd

An automatic with or without air would have the 3.08 as the standard ratio, with a 2.73 as an option, both posi units.

The codes for 3.08 would be AV for a 427, positrac.
AW is listed as a heavy duty posi unit, with 427.
The only 2.73 unit was the heavy duty posi, coded AY, for 427 w/automatic.

Added info for others...If your '69 car was built after August of '69, it would have the diff codes used for the '70 year model.Adams' Apple2007-07-04 09:38:48

Tom,

The car is a manual car with a/c. The rear that is in it now is geared very low and I think it is maybe a 1970. It does not have a stamping on the bottom. I am not clear on the aw code for heavy duty. What would this rear end been for. Is  the AL unit posi or not. My car was built on Oct. 2' 1968. Does that mean that it would have been an AL the same as a 68 or an AT.

Now that we have more info, here's what I have:(your car is an early '69, so it should be coded as such)

427-400HP
4spd-M20: 3.08 ratio standard, 3.36 optional w/out A/C
With A/C only 3.08 available
4spd-M21/22: 3.36 std, 3.08, 3.55, 3.70 opt.(A/C not available with this trans)

Your car has A/C, so that eliminates the M21/M22 trans, leaving you with the only possible ratios as 3.08, or 3.36, from the factory.
Axle ratio codes are:
AW= 3.08 HD posi 427
AT= 3.08 HD posi 427
AU= 3.36 HD posi 427
AV= 3.08 Posi 427

The "Heavy Duty" units were equipped with stub axles that used caps and bolt to retain the u-joint/half-shafts. Standard diffs used straps, with u-bolts/nuts. The difference in the two listed HD 3.08 units would be due to different sources of the posi unit.(E= Eaton, W= Warner Gear, D= Dana)

Are you sure there are no numbers on the bottom of that diff? That would really be strange, as they were all stamped when assembled, even the service replacements. Sometimes, they weren't stamped very deep, and may be hard to see.

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Thanks for the info. The diff. in the car has no stampings. I guess my car would have  m20 with one of the 3 A codes. Wonder what the difference between the AV and AT would be??????????

AT= HD
AV= standard

Where are you looking for the stampings?  It should be just in front of the rear dif. cover on the bottom-