Frank50,
Great to see your post, a few years back when I made the case for the Phillips attachments, it was discounted even though I had owned/parted 32 of these trucks and that my personal had them. I purchased my truck from an original owner family and the plate had never been off, attached with the unique fine-thread Phillips screws you mention. I have seen both the Phillips and the clutch head type on trucks leading up to the rivet era. I believe this was a plant specific install issue and that they most likely could be traced that way now by comparing the assembly facility on involved plates. I have never seen a guide for these attachments in my research. Shiny new rivets have warranted enhanced and problematic DMV/Law enforcement actions which have been discussed here. The 47-55 AD's trucks have no frame numbers so it is either to the plate or the engine depending on state.
1949 GMC Clutch heads:
https://www.flickr.com/photos/advance-design_parts_co/8130478934/in/album-72157631870734114/
1954 Rusty typical rivets:
https://www.flickr.com/photos/advance-design_parts_co/8130552154/in/album-72157631870734114/
My 1950 with screws:
https://www.flickr.com/photos/advance-design_parts_co/8129122101/in/album-72157631867396207/
Copy of title showing both numbers as was the practice in Utah:
https://www.flickr.com/photos/advance-design_parts_co/9035376513/in/album-72157631867396207/
Factory photo 1948 clutch heads:
https://www.flickr.com/photos/advance-design_parts_co/17057160050/in/album-72157651722976780/



1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr