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@ex-pat_scot
The 7 speed blocks have the same total width as the new 10 speed ones. New ones have much narrower gaps between teeth, and thinner teeth, so as to fit in the same overall width.
As the rear mech travel is designed for the same total block width, then the "limit screws" should work fine for a 7 speed mech on a 10 speed block.
Your problem will come when you try and index your downtube levers to the mech and block.
Different generation mechs and levers work with different "cable pull ratios" - ie how much cable is pulled by the lever in one click, and how far across the block the mech travels. (the indexed levers are simple; each click point is set as a certain no of degrees of rotation. The mech travel is more complex, as it relies both on cable length as well as pivot geometry).
Short answer: you will not be able to have 9 or 10 speed INDEXED gearing using a 7 speed mech.
Alternative answer: you could use any downtube lever in "friction" mode rather than index mode. I've done this for many years on a couple of my bikes without trouble.
On a slightly different note: you'll find a fair few time triallists doing similar. They use a whole mish-mash of kit, with bar end shifters (end of aero bars) in friction mode. It's easy to micro-adjust your gears as your hands should always be on the gear levers.