that looks to be a porcelain sign, so maybe even 1960s. That's one of those hideous new shields that CA has used since 1999, so I wonder if the original 80 shield fell down - either the original red, white, and blue, or the business shield - or if it was a 50 shield that fell down. In any case, something must've fallen down, to need a replacement so recently.
It seems like 99 was only acknowledged as a through route in signage installations from about 1972 to the mid-1990s, so this probably dates back to when the interchange was first constructed.
I would bet 99% that it is not a 50 shield that fell down - CalTrans has never signed 50 west along the freeway from about Stockton Boulevard to West Sacramento, even though the road post-1982 has been called that by locals for both directions (which is correct). That newer 80 shield has been up for about 4-5 years as far as I know, I've been seeing it for a while.
Yeah, the brand new as of 2009 "Interstate 80 West" sign at the split between 50/80B west and 99 north bugs me, even though it would have been accurate 28 years ago.