I machined an adapter that attaches to the Unimat M12X1 spindle and has a 7/8-20 thread for a Criterion S-1 ½ small boring head.
I used 12L14 steel 1.25” hex rod for the adapter and the adapter is only 0.850” in height. Most the adapter blank was machined on the Unimat SL 1000 and the M12X1 thread was threaded with a tap and the top few threads removed and machined to 12mm diameter for a very snug fit on the Unimat spindle. I created a M12 threaded mandrel from T-303 stainless steel rod for duplicating the Unimat spindle on the larger lathe. The threaded mandrel was machined in-place on a 12” geared head lathe and then used to hold the boring head adapter for single point threading. A single point insert tool was used to machine 7/8x20 threads to precisely fit the Criterion S-1 ½ small boring head.
I included some photos of making the adapter and showing installed on my Unimat SL 1000 with an additional milling/drilling head (see Homemade Lathe Modifications for more information).