15085 isn't in Grant's numbering system. Is there a four figure number on the action? Likely made in Birmingham for retail by Grant. One of their regular customers probably wanted one for a son who might be expected to grow out of it. Fairly common practice by the big London names.
The 70mm. mark is for any 2 3/4" cartridge that is CIP approved. That might rule out some of the American 1 ounce stuff. Look for the CIP mark on the box of ammo. Like Larry says; avoid the heavy stuff. It wasn't what the 28 bore gun was intended to fire anyway. 1 ounce loads are best through a 16 bore. Lagopus.....
