Are you confusing the Fork tube locking Allen bolt with a Fork,"Drain Bolt"?
The Allen bolts are not drain bolts.On our CX500s that don't have fork leg drain bolts on the side you just take the forks out,tip them upside down.Flush with some thinner oil or cleaner,then put back on and refill to correct amount(135 CC in my 1980 Cx500A type).
These is no need to undo the Fork leg Allen bolts unless you are going to renew the Fork seals or there is another problem.
You can usually get the Allen bolts to re-engage by refitting the spring assembly etc and top cap thus causing pressure onto them inside.
If you look on this parts fiche for the GL Silverwing you will see that they,"Do" have a small bolt for draining in the legs which we don't have,