what exactly keeps the excal's thumbhole stock from interchanging with the regular one?

I dont mind the regular stock but I think I'd maybe like a thumbhole better, but it would have to fit my phoenix, awshucks sent me a link a while back that had some awesome custom made thumbhole stocksfor sale and thats probably the route I'm going to take regaurdless(if they're still even available),but I'm still curious as to what exactly keeps the excal stocksfrom intercanging