Hahaha! Mate, I'm not disagreeing with what Dan's saying at all. Although there's a fairly large difference in what we're basing our explanations on. Much of what he is saying revolves around functionality and purpose, whereas everything I've tried to explain is purely from a practical perspective. As I tried to highlight in my response in the ML thread, the Schwebekreisvisier simply couldn't be mounted on the Linealvisier assembly. As with the single barreled Flak 38 and the Flak 30, the Linealvisier assembly had it's own unique Haltearm that was not compatible for mounting either the Visierkopf 38/40, Schwebekreisvisier or the Flakvisier 35's Reflexvisier in the case of the Flak 30. Hence the later introduction of the Haltearm 38 for the Linealvisier 21 to enable it to be mounted onto the Flakvisier 40 or 40a.
Essentially, the Schwebekreisvisier is just a better Linealvisier 21. Which is simply a developmental issue. It's not at all surprising that the Visierkopf 38/40 was phased out for any number of reasons. Although for the sake of this whole discussion, that's fairly irrelevant.
I must admit I'm a little confused where Dan says
"The difference in the Zeilfernrohr 40 vs 40a was whether or not it could be connected to a fire control system-hence the plug connection in the front of the sight.", as I was under the impression that the plug mount on the front of the Zielfernrohr was for a lamp. The plug on the rear of the Visierkopf was for connection to the Flakvisier, or 'fire control system'.
I think it's also worth being aware of the confusions that arise from referring to the Visierkopf 38/40 as the Flakvisier 38 or 40. There was a very early Visierkopf 38 (which quickly became the 38/40, even in the manual) but there was never a Visierkopf 40.
I probably just explained myself really poorly when I was trying to explain the eyepiece for the Schwebekreisvisier so this pic should put that to bed.
Hope this is beginning to make more sense.
Allan