Have you looked at any of the information from either the NPRHA or PNRA?
Worst case there is a general arrangement drawing of ET-1 in the 2025 Vista-Dome Project (and if you're a NP modeler I highly recommend investing in the VDP). The NPRHA may be willing to sell that particular scan:
https://store.nprha.org/photos-and-drawings/ where the file you are after is "Hoods and Doors - Arrangement R-2505 60225-B 10-28-1959.jpg". While not super detailed, it does give the position and size of the roof panels, although it doesn't have details on latches or other access mechanisms. Given the images of ET-1 in the same folder, it looks relatively close to the drawings although none of the images included are top-down.
I'd really recommend ponying up for the VDP though. Well worth it for the NP / early BN modeler. It's only 130,000 files spread across 300GB...