Howdy
No real good answer for you other than it is not unique. I have 2 MKVIs, one is the pro and another is not. Could be as simple as one came with different "stuff" or maybe they just started marking the MKVI differently.
I am sure that someone out there knows for a fact what it is. Try posting on http://groups.yahoo.com/group/DetonicsCollectors/ and see what they can tell you. Mention that I already struck out for you, hopefully we can both learn something!
L8r
Conley Janssen