VMX60, This is a fantastic find! Any chance of a good side-on pic of RHS of whole bike?
It looks to me like it is one of the "development bikes" ridden in WA enduros by Tossa's dad & Graham Sully. The fact that it was bought from RL's home, not new from the shop would also support that.
In fact, for various reasons, I'd say its the very bike pictured in one of the TW articles & also in Ray Ryans article in VMX #7 (same pic), which I believe was Mitch's bike.
In both captions for those pics it is said to be an MXer, but any Ossa-buff can tell you its not the Stiletto/Phantom top end, & the MX Alron 250s ran down-pipes like the 400s. Most likely an SDR engine in the bike in those pics - just as you've found on this one.
As far as I can see in VMX60's pics posted above, this is the later "fastback" style Sprite frame, not the early one, or the Oz-made frame of the blue one in the TW article posted above.
Apart from Mitch, I've managed to speak w another 2 guys closely associated w the Alrons back in the day & who all rode/raced them. There seems to have been some 'tall stories' told in the press back then ("they were trying to create an image", no doubt of a successful enterprise) & there are a lot of contradictions in the various articles. eg The nos produced were variously said to have been 25, 35, 50, or 60, tho its sometimes unclear whether they're referring to the 250s ("real alrons") or the whole lot. In the ever-present tension between accuracy & expediency in the press, one seems to have come off 2nd best.
However, like Tossa, I'm not sure how much to say in print. So bear in mind these are only opinions & pls don't quote me. (I'd rather avoid a lawsuit) My hunch about how many were produced is perhaps only half a dozen ea 250s & 400s. I hope to be able to give reasons why & point out some of the anomolies in the story when I get a little more info.