Interesting question. My understanding is that the NVT built HL500s were produced in 78/79 and the ProFab frames in the States around the same time. That means they don't qualify as Pre 78 bikes, even though the actual Aberg bike was built in 77. Here's the original VMX magazine article's text:
In late 1975, Yamaha turned the dirt bike world upside down with the introduction of the street-trail XT500C.
A hefty 138kg OHC two valve thumper which dipped into the MX model parts bin for forks and suspension, the XT was quickly cloned as a "playbike" variant, the TT500C, within 12 months. Despite losing 15kg in the process the TT was never a racer, no matter how hard Yamaha's copywriters tried to create the illusion.
An expanding aftermarket industry soon offered everything from alloy swingarms, Mikuni kits, cams, pipes and external oil feeds to trick forks for TT500s, but very few ever won races. At best TTs belched great sounds, threw large rocks and made old guys tell lies about Gold Stars which they never owned.
Sten Lundin, a motocross legend in his own right, was the first European to sample the potential of the Yamaha TT500 powerplant.
He purchased an ex ISDT XT500 prototype from an American privateer in late 1975 and began fiddling to create a motocross project of his own. Lundin started by fitting the Yamaha engine to a Husqvarna motocross frame and then took the process one step further with a lightweight frame created by USA-based ProFab to Lundin's specifications. Profab was already a name familiar to Lundin and his colleague, Torsten Hallman, for their own titanium frames, stands and other equipment for Huskies.
Lundin mated this frame to a special aluminium swingarm and state-of-the-art Fox Air Shox. The bike was to be dubbed the HL500, combining both his name and that of Hallman - the Swedish Yamaha distributor at the time.
By late 1977 the original design had further evolved.
It featured weight saving YZ hubs and forks along with dural engine mounting plates and hardware. The whole package hit the scales at just on 112kg and offered around 250mm wheel travel front and rear. Not content to taint his creation with a soft playbike engine, Lundin turned tuning ace Nils Hedlund loose on his project. The end result saw an 11:1 compression ratio, a larger 36mm Mikuni carb and changes to ignition and clutch assemblies, but with the bottom end of the tough XT/TT powerplant left untouched.
Yamaha were keen to launch their XT500 production model in Europe and realised that kudos gained in international motocross carried a lot of muscle on the sales floor. They saw a window of opportunity and leapt through it in early 1977 when Lundin and Hallman approached them for backing to fund a serious GP challenge with the hybrid thumper.
Former Husqvarna star, Bengt Aberg, signed on.
The HL500 soon earned the nickname of "the Aberg Yamaha" in the motorcycle media. Aberg finally capped off an otherwise lacklustre season with a GP win in Luxembourg late that year, but Yamaha and Hallman both elected to drop the project and would not support a four stroke team for 1978.
However the machine had attracted the attention of Yamaha's own European motocross team boss who opted to create a small production run of HL500 replicas. The Norton factory at Shenstone (UK) was asked to build the bikes using a cosmetically modified TT500 engine and a chassis based on that used by Aberg the previous year.
Although they lacked serious GP winning performance and soonearned a reputation for being harder to light up than a barbecue in Scotland, 200 HLs marched out the Norton factory doors in the first year of production.
The process was repeated in 1979 with a further run, featuring wilder camshafts, improved CDI ignitions and 38mm Mikuni carbs. Minor changes to the swingarm shock mounts upped rear wheel travel to 260mm, while adopting YZ400 forks delivered around 270mm up front.
European production totalling 400 bikes over two years was inevitably topped up in other markets with a surge in aftermarket framed specials. Many claimed either inspiration or heritage from the HL500. In the real world of racing these bikes were often on par or even better than the HL "production" machines, but very few survive to be prized as highly as the two Norton-built models.
