Join Date: Dec 2005
Location: Kitchener, Ontario Canada
Sorry for the delay in getting back to you. I have unexpectedly been away so I don't have my library and files on hand and I certainly don't have RPO's memorized!
I assume you have already decoded your tag and VIN, if not let me know and I'll dive in.
Off the top of my head it appears that the codes you don't know are M40, Y02, Y03, Y10, MG and CH CH. M40 is Turbohydromatic 400 transmission. MG and CH CH I would guess are the codes for the rear (MG) and front (CH) springs. Y02, Y03 and Y10 look like other internal GM RPO's you'll never see on an order form, in literature or published on the net. I have a listing of some but by no means all. So I will look those up.
526 and 4080 - I would have to look to see where your VIN falls in terms of production date, but I wouldn't be surprised if the 526 was the sequence number of your body as it moved down the line at Fisher Body, and 4080 could be the actual production date. The C-1 is easy...that's your paint code from your trim tag.
Now the big question - has anyone ever found a sheet like this? Yes, many people have. Save it, it's part of your car's documentation like a build sheet. Now, not really valuable like a build sheet since it is hand written, but a part of your car's history nonetheless. At various Fisher Body locations, as the cars moved through the body assembly, interior installation and final assembly at Fisher (before moving on to the final assembly at Chevrolet) very often a sheet like this would be placed in the car with a quick reference to information about how the car will be optioned, which will affect how the body gets built. This is by no means the ONLY way this was done, other plants had more formal documentation.
I found one myself under my package shelf, laying under the jute padding on the bare metal. It had far less info than yours, with only the sequence number, the model (6489, Impala, 4-door sport sedan) and the interior color ("60" short hand for 860, dark green) I've linked it here.
There was no pattern to these, they were simply scribbled out by workers with information that was relevant to their particular operations.