Spark plug length

[bikester250]

I posted this in the for sale section by mistake so here goes another trial.
My 250 Mach 1S was converted to twin plugs back in the 60's by H&H.
For years I've wondered about the thread length for the original plug. If you install a 1/2" long plug (say an NGK B7HS) the electrode is pretty deep in the thread. Using a projector plug (BP7HS) helps that a lot with the electrode just coming out into the combustion chamber. 3/4" reach plugs are way too long.
So my question is are the narrow case heads built this way or has this head been altered sometime in the past? It seems strange that the thread length in the head wouldn't be a standard length based on either 1/2" or 3/4" reach spark plugs.
Phil