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Thanks Wookie, time will tell. 
As a addition to the story, I understand that they were bought from a now defunct dealer in Kent. Dealer sticker is still on the side panel. 
The pair were dropped off at the dealer by one of their parents by car along with aload of gear and the first time they rode them was direct to the ferry to go to the  Isle of man to watch the racing. 
A roung trip of some 800 miles. Guess they were (weren't/thrashed) run in by the time they got back. Those were the days. 
Such was their reputation for reliability. 

Congratulations  Warren  and great pick Wookie !!!!!
Grear survivor  and  what a great back story .
DD

Gorgeous bike, great story, and what a treat upon visiting the site!

Holy moly, beautiful bike, and what a great story! 

What an incredible time piece and great story!  Congrats on being caretaker of this beautiful piece of History Warren!

Wow,never mind 1 in a million, here's 2 in a million. Awesome story.well deserved botm. Congrats. 

Wow, what provenance. You can’t beat that! I drool over original bikes. I look for verification of all the questionable details, which way do the engine mount bolts go through, what hardware is used and where. Date codes and where they are stamped. Nothing like an original. European model long rear mudguard and factory install dual front disc. I don’t see metal HT plug caps, we’re they replaced during maintenance? What is the thickness of the front rotors? Reminds me of when a friend of mine was an FFL and ordered us 2 Browning Buckmark pistols, they were delivered with consecutive serial numbers. He wanted to keep the pair together and I agreed. Years later he passed away but unfortunately I never saw the pistols again. 

Congratulations Warren!

EDIT- Warren next time you have your hands on the bike please reroute the float bowl vent lines, also one thing I never thought about for originality is fuel line routing, please if you would confirm for me. It looks like the spigot on the petcock facing forward feeds carbs 1 & 2. The other spigot facing right feeds carbs 3 & 4. Thanks. 

Mikaw, I should see the bike over the summer sometime, being what it is I won't take it out to stretch its legs unless it's a nice dry summers day.
Prior to my mates mate passing he used to have the odd bit done by his local shop. Unfortunately it's a Hardly Ablesons dealer! They replaced all the fuel tubing, including overflows, with ethanol resistant tube so can't confirm routing is 100% factory. I'm sure that kind of information could be found by studying factory workshop manual pictures backgrounds closely. It's likely clues could be gleaned regardless of what the main picture subject might be. Although your thoughts make sense as the natural order of routing as least resistance. 
Discs are 7mm. Earliest stock twin disc model with 5mm discs I know of is 650C1.
It was normal for UK dealers to swop the HT caps from factory metal to Ngk plug caps before delivery in the UK as the metal factory caps arc and cause misfires in the wet UK weather. 
That happened on my green 650 when I bought it new. My local dealer was Paul Smarts Kawasaki and if any dealer was to stick to factory instructions it was that one given his links to the factory and the 900's development testing. 

This month we are continue the “globetrotting” with a sweet Z1B from the Netherlands. This particular “Z” resides with its proud owner WillemZ900. This is the story of a 40 year relationship which has had it’s “ups and downs” but I am sure the “ups” outweigh any “downs” along the way. The bike is a 75 Z1B imported to the Netherlands in 76.7 years later enter Willem with a pocketful of cash and “got to have it” eyes. The bike had already gone through some changes at this point prior to the purchase (KZ1000J forks, clocks and wheels). The relationship hit a bump in the road later in that first year when, following a trip to Italy the cam-chain tensioner broke with all the usual unpleasant mechanical lovemaking that follows such an event. Following the engine rebuild the bike received a cosmetic re-fresh.Willem then continued to use the bike until last winter with only routine maintenance, it was at this point that a combination of the bike being tired along with a cracked frame gusset forced the decision to pull the trigger on a full rebuild (Willem had been collecting new parts for many years knowing this day would come)Willem worked on the bike daily for 4 months throughout last winter ready for the summer of 22, in his words “she now rides like a new bike and is fast as hell”
Specs: 1015CC, 29 Mikuni smoothbore pumpers, handmade RVS racing exhaust, electronic ignition. New piston-rings, new hollow cams, new cam chain, new clutch and Ape mushroom, and lots of other new small parts. Frame crack repaired and powder-coated, stock swinging arm braced. rear wheel changed to16. That’s about it

 

A great pick and a great looking Z
Very Nice !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1
Dave

Stunning Z. Love it!! Job well done.

Great job