Before about 1980 all the ports on the first stages were the same size.  In
the mid seventies power inflators for BC's started showing up.  When someone
would connect a power inflator hose to a high pressure port by accident it
would cause a rupture of the inflator hose.  It sounds like a gunshot!  This
is when the manufacturers started making the HP ports a different size.
BTW, many, if not all, of these old regulators had full flow (no pinhole
restriction) on the HP port.  The good thing is that Dave is quite correct,
youl LDS should have reducers.  They are readily available.

Safe diving,
Cpt. Dale