Seems to be no rhyme or reason to it, as the builds I saw looked to be newer kits. Until last night, I had never seen these longer tubes before.
I prefer the longer tube, sometimes referred to as a stuffer tube. The advantages are less internal airframe volume for the ejection charge to pressurize and keeping the wadding and recovery device on the right side of the center of gravity in the forward end of the airframe.
Is the stuffer tube now available as an option? Not sure how easy or difficult it may be to do a partial rebuild to incorporate one. or, if it is even worth the effort.Yes, motor tube length has changed over the years, although it might also be whatever the kit packer had on hand at the time.
Rather than rebuild I would save it for your next Bertha. The kit in the '70s used an 8.65 inch length of 18mm diameter tube. You'll also want centering rings for a BT-20 inside a BT-60. I think by now you have enough links for our fine vendors that you can search for these items if you want.
Looking at Boosted Bertha maybe as a next build.
