Don't forget that at that point in time, RCA was planning on releasing EVERYTHING on CD-4 with quad mixes, which is why some of those odd ball releases like Carolyn Hester and Nat Stuckey came out in quad. This Spur act was there at the right time and if they were going to get an album on RCA it was likely to be a single inventory CD-4.
I think that's it exactly - when RCA switched to the consolidated catalog numbering system in 1972 (which started with Hugo Montenegro's
Godfather album, APD1-0001) they envisioned single-inventory quad LPs as the future, and everything they released in quad from 0001 to 0100 was single-inventory. I don't have the article to hand, but there was a Billboard article from 1973 with a quote from one of their A&R guys about the unanticipated problem they ran in to - because of the prominent 'QuadraDisc" logos on the covers, record stores were filing RCA's single-inventory quad LPs in their dedicated quad sections rather than in with the stereo LPs as the label hoped for. So for people looking for stereo versions of these albums, they were basically "no-inventory" LPs since the only copies were hidden away in the quad section.
So after 6 months or so they abandoned their single-inventory approach - the last couple of single-inventory quads were Jimmy Castor's
Dimenson III and Jose Feliciano's
Compartments. Interestingly though, even though they went back to dual-inventory stereo and quad LPs, I don't think the label returned entirely to doing dedicated stereo an quad mixes - Mike Dutton told me when he was mastering Mancini's
Symphonic Soul (from 1975) that he noticed the stereo version was a fold-down of the quad mix, presumably because he had both versions lined up in his workstation and could A/B between the stereo mix and the folded-down quad. I don't know if the practice extended to any of their pop artists, but it wouldn't surprise me if the majority of the stereo mixes of RCA's easy listening artists were actually fold-downs of the quad mixes.
Regarding this SPUR single, I suspect there probably is an entire RCA album mixed in quad that never came out - the single flopped and the label probably lost confidence in the rest of the material. I see the production credit is for Dave Blume, who produced all of the Montenegro quads (and the Carolyn Hester S/T single-inventory quad LP) so it's no surprise. I think when RCA switched to the unified numbering system, they probably assigned catalog numbers to projects at their inception rather than at release (so it became sort of like a 'project number') just like CBS did when they went to their unified
30000 series numbering system in 1970. This is why you see albums like Garfunkel's
Angel Clare from September 1973 bearing the catalog number KC 31474, because it was started in mid-1972, when CBS was releasing albums in that numerical range. The recording of
Angel Clare dragged on for nearly 18 months, so by the time the album came out CBS had already released albums like
Beck Bogert Appice (EQ 32140) and Lee Michaels'
Nice Day For Something (CQ 32275) meaning they'd allocated almost 800 catalog numbers to projects between when the album was started and when it was released.
That
BSNPubs document of the RCA unified numbering system document is a good starting point, but it should be taken with a grain of salt as they seem to think that the
PQ8 Q8 series was part of the unified numbering system, which it wasn't - it was the quad tape counterpart of the LSP stereo LPs (and I think the stereo 8-tracks carried the P8S prefix) which predated (and somewhat overlapped the unified numbering) from 1970-1973. I also noticed that Japanese RCA Victor quad LPs from about 1973 onward carry what looks to be a unified numbering prefix matrix numbers like JPD1 so it's possible that the unified numbering system covered both Japanese and American (APD1/JPD1?) releases. It's also possible that some of the unknown/unused catalog numbers contain some unreleased quad LP projects, like SPUR, but while they're an interesting footnote you'd have to imagine that they're probably of minimal musical interest - if RCA figured the album wasn't ready for primetime in 1973, you'd have to imagine that 45 years later (given the relative weakness of some of RCA's other previously-mentioned quad one-offs that did see release like Carolyn Hester and Nat Stuckey) it probably stayed in the can for a reason.