There's a couple of good suppliers in Europe who now supply adaptors for the front and rear threads (M50 x 1) of the PST etalon.
http://www.aokswiss.ch/index_sonne.html
http://www.teleskop-austria.at/shop/...dap-pst-508-hu
The pentaprism/ focus arrangement in the original PST is marginal. An out of alignment pentaprism can cause real headaches with astigmatism. Dump it as soon as you can!
As Peter mentioned: the eyepiece holder on the Black Box comes in two versions, both use two sections....
The section nearest the Black box can be empty (just a spacer) on the original "gold" objective models - this can easily be removed to increase the available back focus.
On the later "blue" objective PST's Coronado added a small "mini-erf" to the front of this tube to reduce the energy loading on the more sensitive blocking filter.
For imaging, this can be safely removed.....
The tube section, where the eyepiece goes, contains the small (5mm diameter) blocking filter. This is a CRITICAL component in the Ha filter system.
Hope this helps.