Ah yes....I remember having to do this once with my GF-777.
Tricky, isn't it?
I think......What I done, was to drop in the antenna without the little wee washers, and to then lift it back out half-way, and gently persuade a washer into the gaps either side.
Once placed in there, ( and yes, it is fiddly!! ) then drop-in the antanna the whole way.
Now then, if you have got the washers lined-up correctly ( again, fiddly! ) you should be able to see straight through the screw-hole and out the other side. Once again, the little wahers can be persuaded into place with a cocktail stick / toothpick ( call it what you want.....) by jiggling the stick CAREFULLY in the screw-hole. Be careful not to snap the stick in the screw-hole!
Once everything is lined-up, just twist in that screw! This can be fiddly also!
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OTHER IMPORTANT TIPS !!!!!
Do this in a place where you're most likely to be able to find the screws/washers if you drop them / ping out from somewhere.
In a bath, WITH THE BATHPLUG IN PLACE is ideal!
Also, if there's ventilation slots nearby that the washers / screws can fall / ping into, then BLOCK THEM OFF with newspaper and masking tape. You don't want to have to crack that box open to recover parts that fell in there!!
Also, tempting though it may be......DO NOT try to get the washers in by bending the post apart to slide them in. Very tempting, but these posts have very little-to-no give in them. Once you have snapped off one side of that post, you'll be kicking yourself !! DON'T DO IT.
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All the best.
Now go and fix your antennae.
-BoomBoxDeluxe