Hmm, that's interesting as I can't find mention of the focussing extension anywhere either. It certainly looks like it's a Kaiser part, but doesn't have any identifying marks. It's basically a flexible 'rod' with a knob on the end that fixes on in place of the centre fine drive focussing knob (my head, being a 6x9, is fitted with the 6:1 ratio fine focussing knob). Hopefully someone else reading this can elaborate on this ?
I do indeed have the longest post. It works well for me, although my only previous enlarger experience was a couple of months using a Minox enlarger so I don't really have much normal size enlarger experience with which to compare it. I'm guessing that with the head up the top one would have to be very careful about head alignment to keep it parallel to the base board as misalignment is going to be more noticeable at the edges of very large prints.
I've got a 6x9 camera so I was looking for something bigger than a 6x7, which seem to be more common. I thought a 4x5 would take up far too much space. The one I have came up on a well known auction website and I managed to bag it and a 105mm APO-Rodagon lens for a very reasonable sum. It's probably a little bit bigger than I really need; I currently only print at 10x8, although it gives me some room to grow into when I want to make bigger prints. It turns out it probably takes up about as much space as a 4x5 but is very easy to dis-assemble and is light enough to move easily whereas a 4x5 like a Devere is more of a permanent installation. I'm not really qualified to talk about the merits of a 6x9 against a 4x5 but those discussions have come up before on APUG so you should be able to find something if you search the archives a bit.
One argument is that if you're using 6x9 it's easy to be seduced by the dark slide and you end up going to 4x5 before long anyway

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Crispin