Wow, here I am, many years after your original post, wrangling with the same frustration!
After starting with likely the same 1954 drawing I was confused -- were the dimensions on the drawing in code, what the heck is going on? Then it dawned on me that just maybe that a 1954 frame is not quite the same as the 1960 frame in my garage. A quick search brought me to this thread and the answer to my question! Thank you!
I'm creating a 2D model of the 1960 frame in a graphics program, to help with creating engine plates, since my 1960 A10 frame is getting a 1969 Triumph T120R unit engine. Converta plates were made for this pairing at one point, but I can't find any and the consistent recommendation from forums is to make your own.
I've attached a cleaned up, transparent background, version of the 1954 frame drawing in case anyone wants it. If my 1960 version turns out I'll post it too.
Thanks again!
Matt
Nova Scotia