Here's a pair of Philips F20T12 blue lamps that I found at the local Restore, they're pre Alto too and have a big drop of mercury inside. The date code on both lamps is a bit smudged though.
That was my first guess but OB would date to Feb 1990 or 2010. 1990 won't make sense since the font is the newer bold font and 2010 also won't do since the Philips shield wasn't used after 2005.
Actually looking at it again and more closely, it looks like it could be Feb 1990 since some Philips lamps had the bold font and I can't think of any other date possibilities.
And Jace what happened to MINE MINE MINE?
I believe there was Shield on earlier ones...I will check my 1987 Philips....