Smitty
In my my life I have been accused of many things but being a Wilton or for that matter any vise expert is absolutely not true
Having said this I believe the bullet vises which many here agree were originally based on the Czech York vises have always had swivels base and in fact I have a number 3 bullet Wilton with patent pending that is probably earliest bullet and that has a base.
Later on I believe Wilton produced the N (no base) and S (swivel base) versions. So your 600 might very well be 600 N. And my limited understanding is that the N and S are the same vise except that former is sold without the base and in principle you can buy a base for it and it will fit perfectly.
Again this is base on my limited knowledge and there are many Wilton experts here that can answer this question much better.
(Now you have put me on the spot and Wilton experts will be all over me


)