The little spherical tank was originally a sand blaster in it's own right. A gun using a compressed air line sucked the sand up by venturi action, and the used sand was sucked back into the pot by an attached vacuum cleaner down the outer of a co-axial tube, the centre of which was the 'sucked up sand' line. The blasting nozzle was surrounded by one of several shaped rubber cups that restrained the used sand. If you look at the 'skirt' of your tank, those holes were where different shaped rubber cups were stored. They were intended for small scale car rust removal - just small spots of rust - the shaped rubber cups were made to fit round things like door edges, and also there were flat ones for spots in the middle of a panel. Tediously slow to operate if doing more than the odd square inch.
I sold mine about 8 years ago
What grit are you using?