CBS started a new combined numbering system for all their sub-labels (including Colubmia, Epic, etc.) at the beginning of 1970 and everything issued after that followed. It was just sort of a happy accident that quad began around the same time (January 1972) so all the quad stuff coincided with the beginning of the 3xxxx numbering series.
Whereas it seems RCA decided to go back and remix a lot of back catalog stuff from before the dawn of quad, it appears that Columbia believed that releasing modern/current stuff was the best strategy for quad to take off - you can see it in the fact that they commissioned quad mixes of stuff like Edgar Winter's White Trash (1971), the first Johnny Winter album (1969) and Laura Nyro's 'Eli & The Thirteenth Confession' (1968) and then decided not to release them in favour of albums from 1972 onward. Obviously there are a few older titles that Columbia did remix in to quad (BS&T II, Paul Revere, Sly's Greatest Hits, the early Santana albums) but they're the exception rather than the rule compared to mixes that were 'contemporary' at the time.
Linda's right that anything remixed for quad from before 1970 got a new 3xxxx designation - the other ones that come to mind are Chicago Transit Authority (GP 8 in stereo became GQ 33255 in quad) and Chicago II (KGP 24 in stereo became GQ 32258 in quad).
The interesting thing (for me) with the 3xxxx numbering series is that there are huge swathes of unused numbers. Were these earmarked for releases that never were, or did they use the numbers internally for other assets like master tapes or session reels, or were numbers just assigned arbitrarily (ie someone wanting a nice round number), or was there some other criteria like numbers being tied to year of release or something else that I haven't even thought of.
I posted a link to
this site in another thread a while back, it's an extensive list of all the known releases in the combined CBS 3xxxx numbering system, both stereo and quad. Even if you just have a passing interest in this stuff, it's worth a quick browse.