That refractor is gorgeous!
As to your question, my first thought was to wonder about the wall thickness of the brass tube. Apparently(?) it is thick enough for there to be tapped holes for the two knobs you pointed out (although I do see two smaller screw heads [?] protruding from the tube near those knobs ... perhaps those are to secure some sort of curved plate [??] or thicker cylinder(s) inside the tube, into which the holes for the knobs are actually tapped ?). Anyway, before planning your dovetail / saddle attachment around those two knobs, personally I would want to confirm that the tube in their local vicinity is strong enough to carry the weight of the tube via those two attachment points. But clearly this must have been the original design intent, so it's probably the preferred way to go! Here's a photo I found on-line of a vintage 4-inch f/15 Clark refractor, which shows knobs similar to yours ---
(ref: the 3rd photo at the auction site here: https://www.skinneri.../2760M/lots/299 )
Alternatives to be considered (in case there is any doubt as to the tube wall strength locally at the two knob locations) would include either (1) incorporating tube rings into your mount attachment design, or else (2) fabricating a clamshell-type structure to surround the tube, possibly using the two knobs to secure one half of the clamshell to the tube, but overall deriving its strength from the clamshell structure and the fact that its gets clamped around the tube itself.
(1) As far as tube rings attached to a dovetail are concerned, yesterday I was doing some enjoyable reading about long refractors, and I was quite impressed by member Don Taylor's re-engineering of the tube rings on his Altair Starwave 102ED f/11 refractor: see the first few pictures in his CN gallery starting here ---
... which he upgraded to ...
(for the associated discussion, see here: https://www.cloudyni...-9#entry9237008 and later here: https://www.cloudyni...bob/?p=10276128 ... see especially post #3 of this 2nd thread for some close-up photos).
(2) Some nice examples of the vintage clamshell or "cradle" design I had in mind would be this:
(ref: https://www.cloudyni...ion/?p=11097433 )
... or this ...
(ref: https://www.cloudyni...ion/?p=11114816 from the same thread).
In both of these two photos, note how the lower sections of the two rings are connected by a curved "cradle" that additionally supports the tube.
Hope this helps,
Edited by jkmccarthy, 04 June 2021 - 12:57 AM.