[Kenth]

This issue has been discussed over and over again, still
NO-ONE has ever shown proof of two different size chambers #48 heads from factory.

#12 heads are the eqivalent RAIII and M/T heads for 1970 and there is only one size from factory on those too.

I have owned several pairs of each and have not once noticed different sized chambers.
#48 and #12 are the same 66 cc heads.

Use them on any size engine with proper piston tops, dished on larger than 400.