Yep, it’s not the Argonaut, but the upside is that it’s not the 150 either. That’s the earlier Synta 180mm (7”) Mak which still makes it a very good deal. They are currently retailing at just under $1300, (for the OTA).
Good catch ... from the length of the dovetail bar, I think you may be correct that it's the 180mm.
Odd that the seller's reply to me last night explicitly referred to it as a 6-inch in response to my inquiry about the scope and its aperture. The single (maybe ~ 11-lb ?) counterweight shown in the picture is not nearly sufficient to balance it in R.A. if it is in fact the 180mm version (which would make it a much better deal, certainly). The visual back and star diagonal in the photo are both 1.25" versions.