3/8 is the length,3/16 width.I beleive that the number after t is a width in 100th of an inch

T-50 1/2 inch

T-25 1/4 inch

T 18 what ever 18/100 of an inch is

measure the slot in the tool to see if the T-18 will fit.

I wish I remembered which Heller took the smaller staples.I pulled my heller E model out of my tool box and I think it is the one that T 18 staples will fit

Robby