Since some folks checking our your auction might read this, there have been a lot of developments in the area of date stamping Wilton vises that might shed additional light on this one.
Going off hundreds of vises included over several years, it's obvious that Wilton sold vises that were cast in Chicago, or cast with old Chicago moulds after the move to Schiller park, and sold decades later, with corresponding date stamps.
As an example, many 2" baby Wiltons that are marked "Chicago" have been found with date stamps well into the 1970s and even early 1980s. Wilton left Chicago in 1957, so they must have had a pile of them on hand when they moved, or they kept using the old mould for decades to make a batch now and then.
Since 2" baby vises, and 6" swivel jaw vises aren't big sellers, they sort of go together. Either Wilton cast a bunch in Chicago and took them to Schiller Park, or they used the Chicago moulds for those models for a long time after the move.
I wrote a summary of what we've learned and posted it on my website (has a link to the original thread as well).
http://mivise.com/wilton-vise-date-stamping/