I had a very nice original 75 series HD rim I recently pulled in and took the opportunity to get some close up pictures and add them to the Original Tires & Rims Used on 1963 & 1964 Cadillacs article at https://6364cadillac.ning.com/profiles/blogs/originaltires I had previously noted the 3 lb difference in weight between the standard rim and the HD 75 series rim.
Below are the pictures I just added:
Example of a 1964 75 series HD Rim:
NOTE H stamped next to valve stem:
Note Kelsey Hayes, and 15x16 and 64 (year) markings inside rim:
Jason, if you have this rim, please let me know.
It has already been shipped out. If you like I can put you on the wait list for the HD rim.