Yeah, that definitely looks like a 1988 or '89 Quantum-5 SPX. The neck heel & mounting plate pretty much give it away. By 1990 (I believe) Modulus had gone to the SPi (Integrated Neck Joint) which has the large chunk of additional material recessed into the body below the end of the fingerboard, and no square metal mounting plate.
Your bass is essentially identical to my Quantum-6 SPX (portrayed elsewhere in this thread) which was built for me in late '88, except of course for the bridge and extra tuning machine. And I do mean identical: at the time the only 6-string basses Modulus made had extremely narrow necks (super-tight string spacing that Phil Lesh liked) but Geoff Gould told me he could fit six strings on the SPX 5-string neck to give me a little more space. It's still the tightest string spacing I've ever seen on any bass, but man am I glad I opted for the wider (sic) SPX neck.
Your bass probably sold for ~$1700 new.
Kermit Driscoll (bassist with Bill Frisell's band in the 80's & 90's) played a Quantum-5 SPX.
Geoff Gould has kindly confirmed to me by email that it is indeed a Quantum 5 SPX.
