Hmm, reading The Manual, eh!
I also notice that the e-TeX manual says
'The additional registers, numbered 256–32767, can be used exactly as 
the first 256, except that they can not be used for insertion classes.'

So what happens when e-TeX reads  \reserveinsert <bignum> {<stuff>} ??

a: An error?  But then what happens to <stuff>
b: Or does <stuff> just silently disappear?

c: Or (a variant on b:) an insertion item gets into the current list but none of the associated registers is created/populated and this item is subsequently ignored when building pages etc. 

Questions, questions, questions ...

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