[old_tv_nut]

I hope you can find a real retrofit kit, but they are scarce. I never found one. I used some CTC-7 pieces plus miscellaneous jury-rigged hardware to put a glass tube in my 5 until I could get a 21AX. Regarding authenticity, there is no visible difference from the front, and you can do it without destroying any part of the 5 (just save a few parts like the plastic funnel and edge magnets in a box), so if/when you get another 21AX, you can always reverse it.