The DOP drop, the on off only works with this rail and chances are your asa would not fit,it has a reverse dovetail not worth the risk m8.The on/off will pay for itself in lost gas and split "o" rings anyway.
How about £12and £7 posted for either, dont forget it will cost a couple of £ to post and paypal fees.
Dont think the price is unfair.
If you want to get up close and personal with your marker, then these babies will do it.Trying to help.