Holy thread revival Batman!
Nope, they are not interchangeable. The cup ones are deeper and go into a through bored hole. The dish ones go into a shallower, stepped hole.
I prefer brass ones but take what you can get, these days.
[edit] Use a sealant, after cleaning the hole out to bare metal.
I use Loctite 515 but Permatex #3 is OK, or good old Stag flange sealant.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

You may just find them at Supercheap or Repco for the welch plugs. They're not mini specific and a lot of cars used them. Might be worth ringing as you will get them a lot easier.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Thanks Doc and Mick.
You never know how an old thread can make a come back.
I thought it was the dish type.
as i have found out though web trawling. You tap them in from the outside flush convex out,
then tap centre to lock .
what is the best tool (size) to hit in the middle to lock in place?

A socket extension bar does the trick for me!

(Now where's my butcher's apron...? I left it here somewhere...)

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.