Originally, the mast, first a tall one then a short one, was attached to the upper aft of the rear canopy section and was lost with it when the canopy was jettisoned. The third section was what was actually jettisoned and since it held the second section trapped between it and the windshield the second section of the canopy then fell away as well. When the Erla canopy came along it combined the second and third canopy sections in a single assembly and so a sideways D shaped tubular frame was made to retain the aft hinge of the Erla canopy and laid flat against the aft slope of the upper cockpit. At the top of the D a plate went aft between the canopy and fuselage, laying flat against the top of the fuselage, and on the outside behind the canopy the mast was installed on this plate. A simpler variation at this point was just installing the mast on the aft end of the Erla canopy itself. It appears it was felt the mast needed to be jettisoned so it wouldn't interfere with bailout. It was then decided that the mast didn't really need to be jettisoned for bailout and it began to be installed on the fuselage aft of the canopy. Then it was decided the mast wasn't really needed after all. A possible factor could very well be the radio itself and it's antenna requirements but I have no insight there at all. Could another factor be the supply of masts themselves? These variations did not happen at the same time at the various factories though production decisions were certainly shared.
The main problem with attempts at standardization was the same thing that allowed so many 109s to be built in the first place and that was diversified production. There were so many small vendors it was impossible to keep production moving along while coordinating a slew of major changes. The K was produced by factories that could make the key airframe parts themselves and those that couldn't essentially made their own version of the K out of G airframe parts (the G-10, and a G-14 is a G-6 made out of G-10 parts ... Well, that's my opinion ... lol.)