MTH RS365SH motor variations F3 ABA

In trying to identify the correct motor part number for a RailKing F-3 ABA diesel,  does the shaft length measure from the motor housing to the tip of the worm gear, or is it just the measurement of the worm gear attachment only?  For example, I can measure that my worm gear is 8 mm dia., and I can measure the distance from the motor housing to the tip of the worm gear as about 23 mm, but if I measure only the worm gear attachment, it is closer to 18 mm.  Which is the spec that MTH uses?   And if anyone has replaced one of these, do you have a part number?  Photo attached. I will also try calling MTH.