I recent aquired McCoy 35 RH. Looking down at cylinder head glow plug hole position is  entered in middle of head. I compared to another McCoy 35 RH and 2nd engine has plug hole slight offset from center. Was there more two different configurations? Otherwise engines look the same. Not the lightening bolt version.

These are a matter of generation/ ownership and overall quality.  They ones with the center plug were made under Dick McCoy's direct overview and are better made and better running/lasting.  When the entity was sold (Duromatic I think) they went to the offset plug and lost quality control.  The 'Custom' versions with the lightning bolt were the worst,   the tooling being quick and cheap I guess.  The photo shows two early versions,  the Dick McCoy pieces.  The one on the right is a little more special in that it came from the first production run.  Note the lack of the flat boss coming up the side of the venturi.  That's first run.  The boss was added with the prospect of making a two-speed,  two - needle version ala the K&B two speeds coming out  then.  Don't think Dick followed through with that but the flat boss stayed in the tooling.

Dave

BTW..  These really are gems.  Stay away from the slant plugs.

Maybe 15 yrs ago Currell Patti on the SS forum took it on himself to collect all the different RH plug locations. I think he found 4 or 5 different ones, but found no difference running on the test bench.

bp