I don't have a spare 12a housing to check this so I'm hoping that somebody knows it or can check for me.

is there a difference in depth between the 12a and gsl-se spark plug hole boss?

 

no.. but later housings , both 12a and 13b can have the bossed ring on the outside to make you use the plugs with a taller collar for the ring spanner
( or turned down spark plug socket )

and so putting plugs with a low collar in these holes can make your socket tool bottom out long before the plug is fully threaded

 

The reason I am asking is because I am making a plug for the trailing plug area as I am no longer running trailing ignition.

 

as long as the socket can reach through any shrouds to it , then the depth is the same 12a and 13b