Almost certainly the earliest version of the Boxford milling attachment. Basically the one shown on the lathes.co.uk site in the first "mounted on the machine" picture below the one showing all the parts.
So far as I'm aware there are three variants. Primarily to accommodate different dial and feed handle arrangements but casting shape also changed. As I recall it the feed screw, dial and handle are essentially the same as that used on the the non power feed (model C) Boxford cross slide.
Yours appears to have the earlier short, screw in dial carrier rather like that shown in the first photograph of a Model C on the lathes.co.uk site. Later versions of that screw in dial mount were longer allowing greater cross slide travel on the lathe. The large dial Boxfords have an even longer dial carrier, further extending cross slide travel, with a two screw fixing rather than the SouthBend style screw in fitting. By far the most common.
The milling attachment followed these changes but, I suspect, not immediately allowing stocks of older style dial units to be used up. The one I had was a late type but I've seen the other styles of dial on attachments that weren't as old as they ought to have been if the fitment followed the lathe use on similar years. However it was always a very expensive unit so odds are batches were made which then sat on the stockroom shelf for some time.
Pretty sure none of the SouthBend clone makers produced anything so inherently costly. Even Smart & Brown went for something a little less expensive.
Clive