I found the 609! at

https://www.sheldonbrown.com/tire-sizing.html

It says:

609 mm Rare Dutch size, 27 x 1 1/2

I wonder why I have so many of the 609s.
Either they are not so rare, or they are, which
is the reason people dump them here...

--
underground experts united
http://user.it.uu.se/~embe8573