I think Preston Petty made it easy for a lot of us to register MX bikes for enduro use in the late 1970's with his enduro headlights and rear mudguard/taillight combos.
In 1979 I fitted a PP headlight number plate, PP rear mudguard taillight combo, bulb horn and mirrors to my 1978 390 AMX Husky and got it through a NSW rego inspection with the std Husky MX pipe. The inspector told me that as they had no records of Husqvarna models ( the pre-computer database era was good for that), he would pass it, but I could get a letter from head office in Sydney any time in the next couple of month cancelling my rego. I never got a letter, and keep the Husky registered for 3 years, riding it in quite a few enduros, including three 4-days. From memory an awful lot of the Huskies ridden in enduros back then were CR's, and only a dedicated few rode WR's.
It was always interesting doing the noise tests at machine examination, I never once had a problem getting the 390 through with the std MX exhaust, but saw an awful lot of Suzuki PE175's with their pipes wrapped in sound deadening lagging tape and using the ADR restricted baffle fail the test. The theory was that the Vibratachs that they used to measure the engine rpm could not handle the "strange" fin rattle of the huskies and would indicate a far higher rpm than the engine was actually doing, meaning that you barely got the rev above idle when that told you to stop. It may of helped that I always did the noise test with the choke on, and stood on the left hand (exhaust side) of the bike to sheild the engine noise from the sound meter, or maybe it was just that there were a lot of official who liked Huskies riders and didn't like PE175 riders!
Not that riding a Husky helped me a lot, as I DNF'd every 4 day I entered, with the transmission generally throwing in the towel towards the end of day two. The most frustrating DNF was the 1981 4-day in Broken Hill where the tranny died on the transport section back to the main control at the end of day 2. I had learn my leason from the previous 4 days, and had borrowed a spare transmission, with the plan being to do a complete tranny swap during the work period at the start of day 3, I even had it down to under 8 minutes to do the swap, but alas never made it to the final control, let alone the start of day 3. That was the last time I rode the auto in competition, as I fitted the engine from my SR500 into the frame a few months later and campained it in the 4 stroke class for a few years. The auto engine still sits in a box in the shed, and hopefully I will rebuild it and put it back into it rightful frame again one day.
CJ