The retaining latch is concealed by the upper plate in your pictures, but I think I see its return spring in the first pic. It may be missing altogether, that squared-off arch on top of the acceptor is a guide for it. With the lower screw removed, pulling back the latch allows the acceptor to be lifted slightly upwards to clear its mounting pegs and be withdrawn from the front mountings. The supporting plates make it tricky, usually moving it to one side (to the left would seem best with this one) to gain clearance is enough. I think the supports top and bottom are/were to resist the acceptor jumping off its mounting pegs if the door was thumped hard. At the time, they (well, their absence) were the sign of a non-uk machine converted using a bought-in door and fabricated wiring.
Data East games like Star Wars only had three coin switch inputs, though, so without intervention I think only three coins will actually work. The few Williams System 11 games with a similar situation, e.g. Riverboat Gambler, did without the 20 pence; it wasn't on the coin entry label, and the factory fitment acceptor wasn't programmed for it. This wouldn't do for my employer, so a 'credit board' was fitted, intercepting one of the machines' coin inputs, and posting all credits according to its own settings for pricing - the machines' pricing scheme was simply set to 1 coin, 1 play for that coin switch. The acceptors from our machines were sent in to have 20 pence added, though when I tried one in a newly-altered game it still didn't accept 20 pence, as it had slipped through without being done.
Ps; I recall blue coloured acceptors like this having red sliding latches, and black ones having green latches. Looking very closely, yours seems to be a black latch, with its rear end protruding through the arch.
