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Thanks all.

So the bottom right is the Si-MX16 that might be good or bad depending on the luck of the draw?

The rest are probably 23k MX16's, although they were originally stated to be 21k - and the different colours are just production variations really?

However, point taken that mini (minute!) PCB cans may be better, as are squared off mouldings on the top of the end bell.  

Like the idea of the Smart Phone RPM checker but neither me, nor my phone, are that smart!   

I know the proof of the pudding and all that ... but curiosity satisfied.   

Guess I've just got to USE them.