This 2.6" diagonal is a 1973 Coulter, made when Cave, Coulter and few others were making GOOD primary and secondary mirrors. I can't test it, but would guess 1/10th to 1/20th wave as with most of their products back then. Pyrex, of course.
The secondary holder is standard 3-screw tilt-head with 3/8th inch mounting bolt (see photo). The coating is original and needs cleaning, and after 30 years could use a re-coat due to normal reflectivity loss with time, as any coating does. There are some light scratches on the coating which are hard to see when looking at 90 degrees, like your eyepieces do. I doubt the scratches would affect the view. The scratches are on the coating only, NOT into the Pyrex.
The least expensive Pyrex 2.6" secondary and holder I find from today's "commercial" suppliers are about $85 for the secondary and $35 for the holder. (NOT Chinese).
Will sell the secondary with holder "as-is" for $75, OR WILL SELL WITH NEW ALUMINUM COATING FOR $100, either way shipped in CONUS. The new coating would only take about 2 weeks, but I would want a "down-payment" ($30) before recoating.
Will only be able to check email about twice/day this weekend. Thanks.