Definitely 97 or earlier and very nice indeed. And in my book the 97 Heckler is retro, it just happens to keep up with modern bikes very well. I still ride mine regularly in a modern spec. But with new rigs in the stable, I am just about to go all retro on the heckler again. I built a custom Heckler from a frame ordered in 97 and it has almost the same build details. The differences are:
Disc tabs: mine has IS disc tabs on the rear swingarm
Cable stops: The rearmost cablestops on the top tube are not staggered on mine but that could be a size thing, mine is a Large with the bent top tube.
Decals: my letters had black centres where yours are all silver
Seat stay brace: mine was black anodised and yours appears wider. What is the travel on the blue frame? Mine was 115mm or 4.5"
All Hecklers of that age have solid rear dropouts no replaceable hangers. Other details that differentiate to look out for:
Swingarm pivot mounts: I've seen older frames where the swingarm pivot mount plates brace the down and seat tube. This was notably so on the Tazmon and I think also on very early Hecklers.
Swingarm uprights: the one here and also mine have smooth outer surfaces to the upright members of the swingarm. This means all machining is on the inside. Older and then again later frames have machined outside surfaces also.
Seat stay brace: AFAIK The bolted on brace behind the seat post changes just a little in many years.
97-98 there was Heckler and a Heckler Superlight model side by side in the range. Sometime they dropped the Heckler and continued with the Superlight only. And then later still the Heckler name was re-introduced besides the Superlight on all new, burlier 145mm travel frames.
Keep us posted on your progress,
Enjoy!!