I also have DW mill but don't recognise the flat faced knob you describe. Maybe a mod. done by the original builder. A photo could help.

I wonder of the "circular knob" is the round boss protruding forward from the main casting??
It's shown holding a laser pointer, on the first photo of a DW mill on Tony's site
http://www.lathes.co.uk/dore-westbury/index.html

If that's the case, and it's on part 312 of the diagrams, mine is used as a mount for the belt guard to prevent your hair getting caught on the 3 way pulley on the main spindle.
The construction manual is a bit ambiguous its wording.

The original Model Engineering article, dated 15th May 1964,  contains the following;

As the purpose of the boss, with its central tapped hole, may be in question, I will explain that it was intended to provide a means of steadying the spindle head, should steadying be needed in specially heavy milling operations. Stays or struts, such as those used on horizontal milling machines, can be fitted from it to the corners of the base casting. So far, I have not needed this provision. The boss may be found useful for the attachment of an optical or other means of setting the spindle truly vertical, or at a specified angle.

Bill

Edited By peak4 on 07/05/2023 11:01:40

Looking at my Mk II and comparing that with pictures of the Mk I whatever the knob was for on the earlier version is absent on the Mk II, so my earlier posting above is not relevant. I stand by the call for a photo though

Brian