Good point Jimmy.
I was forgetting the fact that commercially available does not necessaryilyly mean still being made. It could mean 1 shop has bought 100 pairs - and therefore commercially available beyond the end of the production run. BUt there is no mention of quantity in 23.7, so a shop could attend with 1 pair = that 1 pair is commercially available is it not?
This rule 23.7 seems to be a black hole, I think the only tyres I would be certain of not being able to use are cut and shuts and prototype tyre only made available to that manufacturers team drivers....
(very)
So, if this rule did get tweaked to become specific, it would be a nightmare to police it!