I've picked up the bass now. After seeing it in person and checking a few things I'm really happy with it.
First thing was the neck. Stamped 103-1655 so that checks out for 1975. Also stamped R BUSH which I've seen on images of others.
With the pick guard and bridge remoed, the overall shape of the routing was consistent for that era, based on this thread: https://www.basschat.co.uk/topic/128339-70s-fender-precision-timeline/. Also good to see the through-hole holes were quite small and tidy and the original bridge holes there as you'd expect. The EMG pick up is active with a battery hiding in the main cavity - no scary routing for it thankfully. Battery was dated 2012 😀
The neck pocket isn't quite what I expected. Fully painted, some black and no stamps visible - does this suggest a refin? There's also a odd circular patch - could it have been originally made as for a 3-bolt neck? There no signs on the back of it ever having being 3 holes instead of 4.
The cavities are mostly a similar shade to the main colour, maybe some black and white paint visible although the pickups seem to have tar-like black adhesive underneath. Paint under the pickguard has more of a red-shade - I don't know if the colour changes under the pickguard or only fades on the uncovered part. There's also what looks like a paint imperfection which I'm guessing Fender wouldn't have allowed.
There are no date markings on the pots that I recognised. The pickups have no stamps or markings on the back. Did EMG produce a fully wired set with pots back then?
Pick guard has pencil writing on the back - swirly hand-writing, probably L, LBR, and upper case JMD. The covered hole that might have had a switch has been patched with epoxy or similar on the back.
Frets are quite worn but I used it with the band last night and it felt great. Heavier than my early 80s bass and maybe it bit punchier.
I'm even coming round to the bridge and controls now I see them in real life. I'll probably leave it alone for a while at least.
The first thing might be putting in a passive pickup. I've never had an active bass and the battery isn't easy to change in a hurry.