yeh, those models you are describing was the Z900A4 model of 1976. These were the first model with factory twin discs. I have a Z1A that has the factory option of second disc - and in the spares that came with this bike, was a spare r/h caliper !! These things are like the teeth of hens or the shit of rocking horses.....
The Z1A and the Z1B were not *stop-gap* models

- they were proper models in their own right, both of which had some detail improvements over the Z1. The Z900A4 model was the biggest-selling model in the entire 900 range, but it also had small 26mm carbs instead of the 28mm models, and was the most common model seen back in the day. The prices you see for Z1, Z1A and Z1B models reflect this, as these models were sold in much smaller numbers (especially in Oz) - and therefore are generally worth a lot more than the A4 model.
It just really pisses me off to the max, when I see absolutely insane money being asked by rippoff merchants like Peter Stevens, when these asshats can't even get the fuckin' model number right !!! what a joke......and the sad thing is that some cashed-up old bloke who knows SFA about these bikes, but just wants to re-live his youth - will be the unfortunate victim of this massively over-priced Z1, and never know any better......
If this Z1 turns out to be a Aug or Sept '72 build, with the vaccum balance take-off points tapped into the inlet manifolds, with an engine number inside the first 2000, with every single sticker under the seat in perfect original condition, with an odometer that reads less than 5,000km, with every single OEM bolt in umolested condition, with the correct 4-4 pipes ( and I will bet both of my spectacularly large testicles that the pipes on this bike have date stampings on them ) - with every single rubber drain hose in perfect original condition.....
.......then and only then will I rate this bike as a quality bike worth maybe 25 large on a good day.....
However - I would bet my balls - again - that this bike is just a normal 1973 model, with an engine & frame number up in the 18-20,000 range, with none of the features that give particular examples of the Z1 the *exclusivity* and valued pricetag that these models deserve. You can bet that none of the OEM toolbox would come with this bike, in fact it probably does not even have the tool-tray insert in the ducktail !! If you didn't know that it was supposed to have one, well you would never miss it and the salesman in PS is sure as hell not going to mention it !!!!
