OEM, not aftermarket.  Conn & King, 5/16-24 thread, 5/32 bottom shoulder, lacquer bell screw.  
Fits the following, but contact me if you would like to verify your model's fit.

MODEL BRAND SUBCATEGORY
14K         Conn Sousaphone
2370 King Sousaphone
2341 King Tuba
2360 King Sousaphone
36K         Conn Sousaphone
2341        (Old Detachable Bell Version) King Tuba
B1111 King Sousaphone
1529 King Sousaphone
2350        Body Lacquer King Sousaphone