I realise you are sorted, but i'd just like to add to this business of the plastic reservoir.
I'm still sure it was never fitted from new, the French blue car in your illustration is a series 1 car, ie Sub VA 15000, as that colour was only available till then. Series 1 cars ALL had the solid master from new. I myself own VA26140, one of the last few hundred pre dual circuit series 3 cars and that also had a solid master from new (and still did when I bought it in 2012) So there's really no room for this cylinder to be a factory install.
The only reason I can think of to explain the sizeable scattering of plastic reservoir cylinders throughout the series range/timespan is that the plastic reservoir cylinder must have been an update/supercession for spare parts supplied aftermarket. Either though Leyland/Unipart or by Girling. So if an original failed, this one would have been issued instead. Which at least gives it a kind of legitimacy!
Are you sure about that Steve?
All the Dolomites I've owned have had plastic brake reservoirs, never seen one with an alloy one?
Modified Dolomite Sprint MSO 662P