Hello,
I can't really add much to your knowledge other than to say the P-H 1100 my father had didn't have those cut-outs (but his was in .308 Win).

It's tempting to think they are either for adding weight or cut to remove it - however I don't recall reading in any of the P-H adverts anything about a 'weight system' (I'll check again tonight in an old P-H catalogue) - on the other hand the removal of such a small amount of wood on a heavy rifle is going to make little difference overall.

As far as recoil was concerned the 'African' models had the 'cross pin' & the third stock bolt to the barrel (as shown in your photo) so the slots are nothing to do with a recoil lug.

Unless anything turns up I'd just forget them.

Regards
Webley