@Bob: I just wanted to add a keyword making life easier for Michael looking for 'Michael'.

@All (well, maybe only many): if you open a Voyager or an earlier HP calculator, you will find individual keys. Each of them sits in its own little niche allowing for the famous rotate-and-click operation. If you open a Pioneer or any newer HP calculator or a DM42, on the other hand, you will find an entire sprue connecting all its keys - rotate-and-click became bend-and-click actually there. Advantage is easier (simpler, politely: more cost-effective) production and assembly, disadvantage is the impossibility to exchange individual keys.
So, if you want something like you sketched above, you have to provide a mix of both designs: two niches for the prefix keys and one sprue for the other ones. If you want full exchangeability of all keys, you have to make a major design change leading to a far more difficult assembly - and I doubt you'll be willing to pay the cost of that.
But this is only my personal opinion, YMMV.