Thanks everyone for the excellent input. I'm returning it to the seller, who understands the issue, and he'll be contacting Astronomics.
Sorry for the late reply to this... That's great news... And, what I post below is educational only. You can cheerfully ignore it if it's not of interest.
Below is a star test from the bench on a 110mm F/5.6 oiled triplet. The AT92 should, on paper, be better. Its a smaller aperture and the air gap gives the designer another degree of freedom in which to dial in correction. The oiled triplet is F/5.6 all-spherical design which leaves a lot of high order SA in the star test.
You'll see some similarities between your AT92 and this 110. You were correct to send it back to the original owner.
Images below are of the same 110mm APO at various point of defocus.
Edited by peleuba, Yesterday, 03:01 PM.