I have an older model of a richfield viewer (I dont know the original manufacture , could be Celestron or another) I found that this adapter screws apart and accepts a t-ring as well as screws right into the short section of the ETX 125 photo adapter.
The one Im selling (lower in my photo) does not include the diagonal back as shown in the upper part of the photo (I have two of these and anm keeping one for now). I have played with these on an ETX 125 and on an SCT and they work pretty well (In my humble opinion). there may be some vigneting if used for photos . I believe the lens is 50mm an its 1 1/4 inch format. I do not innclude the etx photo adapter. The glass is clean, no marks some dust, overall setup is in great shape.
My price is $45 shipped in conus. MO of check to rated astromarter.