This is not so much "faulty" as non-standard and requiring some special notes for functional installation. Moss Motors part number:
163-606 "VACUUM UNIT, as original (with threaded vacuum fitting)".
On one Moss web site page a picture (above left) shows it with the adjuster nut included (but not noted as such in the description). On another web page the adjuster nut (and associated parts) are listed as N/A. In the printed catalog the nut and associated parts are listed as "Vacuum Advance Unit Adjuster Kit" part number 163-738. This is shown as a 4-piece kit on the Moss web site (above right).

The problem is that the thread for the adjuster nut is non-standard, requiring a non-standard nut, but that little detail is not mentioned in the catalog listings. So when you buy the vacuum unit, it cannot be installed, because the original adjuster nut does not fit on the new male thread. The original thread is #10-32-UNF. I haven't figured out yet what the thread is on the replacement part, but the thread pitch is definitely different. If you buy this Vacuum Unit you will need to also buy the matching Adjuster Kit. Then you should also enter the appropriate non-standard parts notes in your car's log book, so you don't become the future DPO. Additionally, I don't know yet if the replacement nut in the Adjuster Kit has the original 10-32-UNF thread, or a new odd thread that should fit the replacement Vacuum Unit. (Research in process).

If I received this part, and it didn't fit immediately out of the box, I would send it back for a refund. That may be the only way to encourage the vendor to correct the part (or at least put the appropriate notes in the catalog).