From the link:

It also features a removable pistol grip cap (in horn) with a replacement front sight wrapped in paper inside (this wasn't mentioned in the auction text BTW). You press the small button and turn the cap 90 degrees and lift it up, I haven't seen this on any other rifle.

From Kiwi_bloke's reply to above:

My 1927 M.1910 in 9,5x57 M.Sch has the same arrangement of a removable horn pistol grip cap with the same sight wrapped in newspaper.

And:

The M.1910 has a steel buttplate rather than a pad and this has a couple of spare rounds and a cleaning rod (take-down), under a trap. It also has a Baillie-Grohman peep sight that folds down when the bolt is cocked and then back up again.

My 1922 proofed M1910 Take Down Model with 60 cm barrel also has the grip cap storage and folding peep sight, though I had not before heard of the sight being referred to as 'Baillie-Grohman'.

As I'm sure you're aware by now, the steel trap door buttplate with storage was 'standard' on the pre WW2 Mannlicher Schoenauer sporting rifles and carbines.

The 1939 Stoeger catalog (US) listed the 'special folding peep sight' as a $12.00 cost option, as well as the option of various barrel lengths for $15.00, but not the removable grip cap. The 'Takedown Model' also carried an additional charge of $15.00, about the equivalent of today's $298.48 USD:

The 'special folding peep sight' and cleaning rods stored in trap door butt plate are shown in image below from '39 Stoeger: