First off, nice job on the restoration. Your patience is a virtue.
As far as the missing keyway and date-stamp, I think I can help you with that. Before I became a GJ member, I recall being a lurker and was following a Wilton date-stamp thread. It was very interesting and had a lot of good energy from the many participants. Long story short, GJ member "Bluebolt" started a database to make better sense of all the data coming in and I kinda got hooked into expanding on what he had.
Your Wilton 400S has it's original badge, which would be on keyway date-stamps from (1979-1995).
But wait, there's more. Wilton machinists vises with date-stamps from (1979-1984) had the date-stamp also cast on the moveable jaw, drivers side. Your vise has the extra casting and I'm sure it's the same date-stamp that would have been on the keyway as well, since it is consistent with my very expanded "Bluebolt" database.
I would bet my neighbors farm that your vise had a date-stamp of "4-81".
Let the controversy begin!