Photograph of a PST etalon module, rubber band removed:-
The two sides have the same thread size which is M50*1, one side male and one side female. Therefore, the PST golden tube can be screwed into the PST black focuser body directly without the PST etalon module, that indicates the thread size is identical.
In order to use the PST etalon module as an internal module, the following adapters have to be made:-
One of the adapters is to allow it to attach to a 1.25" focuser, that side is originally attached ot the PST golden tube, facing the objective.
The other adapter is to make the other end (originally attached to the PST black focuser body) to accept 1.25" eyepiece or any camera with 1.25" nosepiece.
There two adapters should be able to screwed to each others without the PST etalon module, by then it will become a 1.25" extension tube, with a fat body. Since the male thread side of the PST etalon body has a protruding part, if PST etalon modules are to be double stacked in the future, empty tube with M50*1 thread should be made to connect them together despite they have identical thread.
The whole chain of adapters with the PST etalon will eat up 7.2cm+ of back focus.
[Additional Remarks on 30th of March: adapters are being made... I guess they will be available after my trip]
Last night, we also found that older version (orange objective) and newer version (blue clear objective) PST etalon modules are not very identical, they looked so at the first glance, but if you hold it towards a bright light source, the old version will show very clear newton rings but the new version will not, we don't know why, however.