hi, maybe worthwhile to spend a bit of extra attention.
BR-M600 are to me a across-group brakes
used in LX group (pic from 1999)
used in red with -MX as some kind of suffix (pic shows DX group ~2002)
it was intended for BMX use, with 110mm hubs.
used in earlier STX-RC (pic from 1997)
Slightly different, but still very similar, are the BR-MC40 from later years of STX and STX-RC groups (pic from 1999)
They have same generic shape, just the BR-MC40 has a different brake shoe holder.
While BR-M600 need pretty specific brake shoes with screw like used at U-Brakes, the BR-MC40 is using regular V-Brake brake shoes like used on the XT with parallelogram (and then got standard for any kind of V-Brakes)
Also the Brake levers had been identical for all the mentioned groups, BL-M600 or 601 (w/wo MX) and BL-MC40 are identical other than color or finish.
To me those are quite good brakes, powerful, easy to adjust and reliable, much more durable as XT or XTR ones of that generations which are prone for issues with the parallelogram.