I suppose you could try a touch of grease on the 'knobbly bits', then rotate the caps a few degrees if you can, to get them out of the grease. Tony
Going to try a rubber mallet first as i suffer with my hands now a days cant hit it as hard as i like
So went outside to get some pics as im not sure but they might be 67 year only but hope maybe @davidoft Might clear up this is it fitted at bottom its close but not