Maybe before the period you're interested in, but I have a few old Working Timetables for the Slade Lane - Piccadilly - Oxford Rd - Cornbrook Jn. section from the 1970s.
These give details of platform allocations, ECS moves and scheduled shunts at Piccadilly.
They confirm there were no scheduled passenger trains from Platform 12 in that era, but it had regular, limited use for parcels.
May 1977 - May 1978 TT shows the following moves in Platform 12:
- 1607 arrival of 4H10, 1510 Oldham Clegg St to Piccadilly Parcels.
This appears to shunt to Plat. 11 at some stage, then depart from #11 as 1M25 1808 Piccadilly to St Pancras Parcels.
- 2322SX arrival of 5E66, 2312 Longsight to Piccadilly ECS. This is scheduled to shunt from Platform 12 to 11 at 0010 and form 1E66 0120 Piccadilly to Cleethorpes News and Parcels.
- In the 1973 timetable there was an empty EMU scheduled to arrive in Platform 12 at 0005SuO from Longsight after servicing. There is no corresponding departure for this unit on Sunday morning, although the incoming did carry an "Alderley Edge via Stockport" headcode (5H52).