OK ... a bit more research leads me to think you are correct ... it seems that some 27s, maybe those made in Germany, indeed did have a pin to hold the piston washer screw in place and advice was to drill it out. See Idiots Guide posts 7,8, 10

Chambers show the Original 25 that has the pin. If the pin goes though to the other side there should be no problem, if it only goes in one way then drilling it out may be the only option?
Cheers, Phil