<VV> Highly technical question

[BobHelt at aol.com]

According to Bob Kirkman's Corvair cylinder head chart  in the Tech  Guide, 
Corvair heads for each hp were made for 1962--64 that had two different  size 
holes drilled in the right hand side of the head looking at the rocker box.  
Some heads had two 1/4-20 holes and others had two 5/16-18 holes. (not the  
3/8-16 holes!)
 
I know why this might be for the 140 hp engines because of the LH muffler,  
but I'm talking about the EMs (esp 1964s).
 
Does anyone know why there were two different  heads of each hp with  
different hole sizes? I understand the purpose of having holes there. Just why  two 
different sizes?
 
Thanks in advance.
Regards,
Bob Helt