Prototype set is here

[John Folsom]

Nick, The earlier tubes had a 14 pin stem and base. I would speculate that this was because they had a 14 pin stem and base in the inventory. Later on, it became obvious that additional electrical isolation was needed between the convergence pin at [edit] 7-9KV and the other pins, so a 20 pin stem and base were developed, allowing 3 blank pin locations on either side of the convergence electorde.

I just checked my Sparton metal bell CRT, and it is a 20 pin 6 bolt CRT just like yours, with a type number of C73547, just like yours. My bell rim number is D255, and I believe Pete said you rim number is D205 (?). This all seems to make sense. My Sparton set is a early 1953 NTSC set, as best I can tell.