Have you checked the other sides of the ballast? There's no date code on this label you are showing here. BTW the Unidor was made between '70 and sometime after '77, maybe as late as '80.

Ive seen some of the older Westinghouse ballasts stamped before

My '73 400w MV ballast that is in my GE is stamped. I don't know what it says though because I have to take the ballast out of the fixture to read it. I know for a fact though, there is no Company expressed on the ballast at all. I'm assumeing it's a GE because it came with the GE M-400, which is NOS.

My ballast on my newer ITT Model 25 FCO is Stamped as well.