Headlamp Bulbs

[wings515]

I have a 39 PT81 pickup.  I plan on changeing the headlight to Halogen but there are two different style bulbs that can be used.  Basically the filament on the original bulb is either a C6/C6 or a C2V/C2V.  The difference being the position of the filaments in the Prefocus base.  The C6/C6 has the filaments side to side and the C2V/C2V has them top to bottom.  Each has its own merit but right now the bulb that was in the truck when I got it is the C6/C6 type.
Some documentation state the key for installation is offset in the one of the bulbs but I can not find a drawing that shows the difference.
Does anyone know what is the correct bulb and orientation?
It appears an upgreade had been installed in the headlight housing because I have a number 2531 bulb installed.  I think the original was a 2330.  This is much lower in wattage and candlepower.

Thanks,
Dan