D12-2/136 Cylinder heads ??

I was a little mystified by this part number appearing in some discussion on the NOC,
and also it seems in DynoDaves cylinder head page.

It appears it should be R12-2/136.
More in keeping for 1960 than 1949 !

P.S. Was there a R12/136 ?
Or, what is the reason for the -2 in its part number ?

This thread may be dead but
Norton parts manuals use 12-2/ to indicate when part was first introduced on model 122 eg
H12-2/165 main tube (short roadholder) on Model 88.
Sometimes written as H12-2 and sometimes as H12/2

Bailey