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Craig D you wrote:  “Looking at the Hughes, Husun, Hughes & Sons and Kelvin Hughes sextants, they all apppear to use the same Galilean  scope designs and parts.”

They do don’t they.  However, close scrutiny of photographs do reveal one important difference between scopes on Hughes sextants.  Some have a normal continuous thread and some have interrupted threads, presumably for a quick change.  I’m no expert, but if you’ve got interrupted thread scopes you need an interrupted thread collar, or there’s no point in having the interrupted thread.  My ‘Mates’ sextant has continuous threads on both the scopes and on the collar.  Whether you can mix and match continuous and interrupted thread scopes and collars at the expense of giving up the quick-change facility, I know not. 
You should be able to measure the diameter and threads per inch you’re looking from the parts you already have.  If not, I could measure mine for you.  DaveP