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Cryt58, I will offer up what I know about 61 models. This may help. I am not sure how it will correlate to your 58. First I believe your casting number is 3725899,may not be visible from your viewing angle. The other digits I believe to be B78 which loosely corresponds to February 7 of 1958 which was the day the housing was cast at the foundry. The circular metal tag is a indicator of the gear ratio of the installed gears. A completely round tag (as yours appears to be) indicates a 3:36 ratio. Now keep in mind I am basing all this information on what is the right stuff for a 61 year model. On the side opposite of where that casting number appears the may be stamped, not cast assembly code. Many time’s difficult to see. I am uncertain if they stamped them in 58. If they did it will be the key to telling you a lot about the originality of the unit and what engine ,transmission etc. was installed at the factory. You really need a guy well versed in 58s to help you . I am sure someone more specific and experienced in 58’s will provide feed back..Good luck Have fun!
B73 translates to feb seventh 1958
the round metal tag indicates 3:55 ratio. If the tag had an extra round hole it would 3:36
the tall nuts with washers are correct. May be copper
washers.
The lack of a large “J “ on the casting indicates non posi
The unit apoears to be unmolested
I may have a dated 370 posi diff assembly if it interests you.
Last edited by drainbrain; Dec 9, 2023 at 11:58 AM.
B73 translates to feb seventh 1958
the round metal tag indicates 3:55 ratio. If the tag had an extra round hole it would 3:36
the tall nuts with washers are correct. May be copper
washers.
The lack of a large “J “ on the casting indicates non posi
The unit apoears to be unmolested
I may have a dated 370 posi diff assembly if it interests you.
The tag indicates 3.36, NOT 3.55.
I have fought with NCRS and the aftermarket vendors to change and acknowledge that the advertised types of tags are reversed for 3.36 and 3.55 tags. FINALLY, NCRS has acknowledged the needed change in tags, but the vendors refuse to make the change. Thus, the reproduction 3.36 and 3.55 tags advertised by the vendors is WRONG!
Here is a corrected picture of the different tags.
The casting number (3725899) is for a 57-62 (Corvette) and 57-64 pass car NON-posi case.
The posi cases have a big P next to the casting number (57-61 3743833 and 3789812 62-64). VERY, VERY EARLY 57 posi cased did not have the big P.
The tag indicates 3.36, NOT 3.55.
I have fought with NCRS and the aftermarket vendors to change and acknowledge that the advertised types of tags are reversed for 3.36 and 3.55 tags. FINALLY, NCRS has acknowledged the needed change in tags, but the vendors refuse to make the change. Thus, the reproduction 3.36 and 3.55 tags advertised by the vendors is WRONG!
Here is a corrected picture of the different tags.
The casting number (3725899) is for a 57-62 (Corvette) and 57-64 pass car NON-posi case.
The posi cases have a big P next to the casting number (57-61 3743833 and 3789812 62-64). VERY, VERY EARLY 57 posi cased did not have the big P.
I was hoping Mr Parsons would chime in. Consider his
words as gospel.
The tag indicates 3.36, NOT 3.55.
I have fought with NCRS and the aftermarket vendors to change and acknowledge that the advertised types of tags are reversed for 3.36 and 3.55 tags. FINALLY, NCRS has acknowledged the needed change in tags, but the vendors refuse to make the change. Thus, the reproduction 3.36 and 3.55 tags advertised by the vendors is WRONG!
Here is a corrected picture of the different tags.
The casting number (3725899) is for a 57-62 (Corvette) and 57-64 pass car NON-posi case.
The posi cases have a big P next to the casting number (57-61 3743833 and 3789812 62-64). VERY, VERY EARLY 57 posi cased did not have the big P.
What is the cast andcdates for the370 posi noted below?-Thanks Shawn
The lack of a large “J “ on the casting indicates non posi
The unit apoears to be unmolested
I may have a dated 370 posi diff assembly if it interests you.[/QUOTE]
One thing for sure, it is a NON-posi case.
ALLLLLLLLLLLLLL 57-61 POSI cases had the 3743833 casting number, beginning about mid-Dec 56
ABOUT Jan 57, an X was cast next to the casting number.
Then ABOUT late-Feb or early-Mar 57, ALL posi cases got the big P which replaced the X.
In 62, the casting number for a posi case changed to 3789812.
ALL 57-64 NON-posi cases had the 3725899 casting number.
In my book, ANY 3743833 case with NO X or P is a high value part!!! That would be correct for an early production 57 (Corvette or pass car) with factory posi.
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