I think you've covered a few bases here Geezer.

I picked up a 52 from Litts around mid 89 and this was a Firebird variant. My memory is hazy now as to those times but back then I was very much an HW officianado and I was curious as to the upstart challenging the 80. So after a relationship breakdown I treated myself to a Firebird. To my mind the only marque that should have worn that moniker. Thinly veiled dig at BSA there for pinching the branding a few years later
Anyhow this was my first Diana (it was branded as such then). It struck me as equally well built as the 80 with an air of solidity. The trigger couldn't match a Record but was perfectly adequate. The foresight back in those far off days was a hefty metal casting which I later changed over to a mod the make of which now escapes me but which resides on the gun to this day.
I concur with the early springs. Later, on stripping the 52 down, I found a quite diminutive spring inside. Clearly to throttle back the 52s ease at making power.
Despite me thinking I had picked up a reasonably early model of this gun I have found in more recent years that Original branded models exist clearly pre dating that of my own. They must inevitably be a rarer version of these models.
The 52/4/6 model guns are very well built and can achieve excellent accuracy given a matched pellet. I'm a convert and now use a 54 for much of my hunting. It's spoiled me and I'm loathe to go back to anything else nowadays.
Dave