What’s with the 1 on this George Foster Mets jersey?

I was minding my own business on twitter when this caught my eye…

Dave Kingman & George Foster at good ol’ Shea. pic.twitter.com/08Iwqp9tXH

— Sports Paper (@SportsPaperInfo) June 21, 2016

There. Do you see it?

Typically a Mets 1 doesn’t have the bottom crossbar (or whatever the term is called).

If you do a Google Image search for “Foster Mets” you’ll see all kinds of shots with a 15 as I would normally expect. There are also a few with this oddball 1.

What am I forgetting?

Also of interest is this picture showing Foster in the rarely seen blue with the NY crest.

EDIT: Greg Prince (Faith & Fear) says it’s 1981 road font.  1981 road font? Why would they have 1981 road font?  The old road font had that kind of 1 (look up some Seaver stuff)….so OK why the orange Mets then.  Were we not doing the gray lettering on roads yet?  Thanks Greg!