I have several pb science fiction books from the 50s and 60s - most have no ISBNs!  In good shape except pages are yellowed somewhat - not so much that the page flakes when touched or anything!  Anyway, definitely still readable, and probably rare now.

Are these postable?

I think yellowing pages are part of the aging process and if the book meets posting criteria otherwise, they should be fine.  I've received several older books with yellowed pages and it wasn't a problem for me.  If someone doesn't like that, they should have an RC for it.

Yes they are postable.  From the help doc:  Search for:  Book Condition Criteria for 'Swappability' at PBS

General remarks:

• This is a used-book swapping site; books are not expected to be in like-new condition.
  • Older books may have some page yellowing or spine-creasing or dogeared pages; these are all OKAY.
  • Decirculated library books may have various markings and/or stamps inside or outside or on the page edges of the book; this is OKAY.  These may not obscure text.
  • BookCrossing books may have stickers or stamps or markings on the outside or inside covers or page edges of the book; this is OKAY. These may not obscure text.
  • Hardbacks do NOT have to include their dustjackets.
  • Audiobooks MUST be the original cassettes/CDs, but do NOT have to include the outer "box" that held the set.
  • Audiobooks must be playable and have all the CDs/cassettes.
  • Books that require batteries to work do NOT have to have working batteries.
  • "Playaway" books must include working headphones.
  • There is an exception to general condition guidelines for textbooks; see below.
  • There is an exception to general condition guidelines for cookbooks; see below.
  • There is an exception to general condition guidelines for travel guides; see below.

• We do NOT have a place for members to describe the condition of a particular copy of a book.
  • ALL books on PBS are expected to be in "good, not excessively worn" condition
    • note the specific cases for textbooks and cookbooks  and travel guides.
  • Books are requested in FIFO (first-in, first-out) order
• If you would like your books to be in "better-than-PBS" condition :
  • You are responsible for setting Requestor Conditions on your account.

I've posted some older out of print books.  The way I handled it was to choose the latest mail date and to send a PM.  In the PM I mentioned that the books were yellowed with age and offered to cancel the order if they PM me before X date.  After the date passed, I mailed the books.

None of the books were actually cancel. 

I do feel different about posting a yellow book that has been reprinted.   But those 40 year old paperbacks can be very hard to find.

ETA  On some of the older books, the ISBN is either on the spine or on the copyright page.  And it will be the shorter 10 digit ISBN.

ISBN numbers didn't exist prior to 1970.

Yellowing is fine -- particularly in older, harder to find books.  You may have a better shot at having them requested if you scan in the covers, too.