I'm not going to answer for John, but here's what I've found so far...
The THREAD ON YENKO.NET
shows an article from the February, 1967 issue of Car Craft refrencing coil spring spacers being placed in a 67 Camaro due to the addition of a 427. The EBAY AUCTION
also referenced in the thread is for a P/N 3789664 coil spring spacers.
I looked in my 1971 issure P&A 26 "Dealer Parts and Accessories Price Schedule" and sure enough, P/N 3789664 is listed under Group 7.425. I looked in my 1968 P&A 34 "Parts and Accessory" manual for the Camaro, Chevelle, Nova and Corvair, and there is no group 7.425 listed. I also looked in my P&A 14a for Corvettes, and again, no group 7.425. Therefore, you can safely say that these spacers were not used on any of the above cars from the factory. I then looked in my 1957 issue P&A 30 for Passenger Cars, Corvettes and Light and Medium Duty Trucks, and there WAS a Group 7.425, but it didn't show that part number. The part numbers shown were earlier than the 3789664, but not by much, which tells me that that particular part was used in the late 50's or early 60's. Unfortunately, I don't have a P&A 30 from the 60's, so I can't verify this, but part numbers starting with "378" are late 50's/early 60's issue.
Now, with that in hand, the group 7.425 title in my 1957 P&A 30 is "SHIM - SPACER, Front Spring", but gives no further explaination. There are a few thicknesses listed (see the picture below), so we at least know now that this spacer was available from GM. HOWEVER, it was never used on the 67 Camaro except in the case of this particular Nickey conversion. I'm assuming the reason they used the spacer is because when this article was written, the 67 Big Block hadn't yet been released so there were no springs available for it. Nickey used a truck (or possibly a full size car) spacer to raise the front end an inch. I would also assume that when the big block DID become available, and therefore the accompanying springs, this spacer was no longer needed and wasn't used.
Remember, magazine articles are written 3 to 4 months prior to the cover date, so no big block Camaro's were yet available. I would imagine that this particular article was written in late September or early November, 1966 before the release of the Camaro 396.