Hello, so as usual nothing is simple with this stuff and its easy to find contradictory info on the web, so inevitably its always a case of a process of elimination to get to a best guess.
On the face of it (just looking at the frame number alone) I’d say your frame was almost definitely produced in 1974 (see attached number chart) although this number chart is only the result of myself and two other members here (CBguy & dwscrimshaw) collating various “marker” frames, these are bikes/frames that have had very specific details/paints/graphics/production runs to help us hang a framework together for dating. If your number had been a few hundred away from our predicted 1976 then it would be in more of a grey area, but 2000 pretty much sticks into late 1974, maybe at a stretch 1975.
Ok, next… the only real 1976 marker we have is the 531 double butted forks only appeared From June/July till about November that year as a frame set in an attempt to shift these 531 plain gauge frames before the Electron was dropped from the range,
During the Electrons long production life I think Holdsworth used a number of different suppliers of plain gauge tubing, but the bike wasn’t advertised with 531 till later in its life, and this in itself isn’t really a marker as pretty much all of these plain gauges had seat pins around 26.4mm (I think)
From my collection I know the plain tubing in the cheap mid/late 70’ took a sub 26mm seat pin and that stuff most definitely wasn’t 531.
The best marker would be if you can remember if the bike had a Reynolds double butted fork sticker before the refurb.
In conclusion, my best guess would be your frame was manufactured in 1974, but may not have been put together and sold until 1976…….
PS, very tidy looking build BTW