[jeffschevelle]

Part # 3819910 was added in the Z28 section of the AIM with date notation of 3/8/67. The GM parts book from back then says that is a 5/16-18 x 1-1/4 bolt.

In my experience all 64-67 upper and lower alternator mount bolts were cad plated. They establish a ground, and typically cad finish was used in grounding applications rather than the more commonly used phosphate and oil.

Can't help you with the head marking for Part # 3819910.

Unless the addition of Part # 3819910 to the AIM was a late issued "correction" for a Z28 (and 3819910 was actually used the entire time on Z's), I would have to assume it was the regular production bolt before that change (dated 3/8/67) was made.

The regular production bolt is 5/16-18 x 1. Although I have seen one totally original 67 Chevelle with a longer bolt there (don't recall if it was 1-1/8 or 1-1/4).

The most common head marking I have seen on the regular production 1" long bolt on Chevelles is:
- RBW, with no hash marks, and with the letters out near the edge of the head of the bolt.

Others I have seen on unrestored 66-67 Chevelles are:
- RBW with three hash marks that meet in the center of the bolt (letters still out near the edge)
- A in the center with no hash marks
- NAT in the center with three hash marks outside of that.

The one that was longer than 1" (found on a July built Baltimore 67) was
- RSC with three hash marks, with the letters between the hash marks.

Happy hunting!