There are various OB 1975s:
All fetch around the same price: €170 to €200
OMC 1972, 28years old I haven't seen any sold since late 2005, but the prices have been steady during last year at around €220 to €250
I haven't seen the 1974 sold, but the 1975 OMCs have achieved a little less than the 1972 bottles, around €180 to €195
So I would expect the 1974 to be around €190 to €230
Obviously the Single Cask 1975's, of which there are many different ones, normally achieve prices around €400 to €450
If you have a set price in mind, I may be interested.........