Assuming you've not already smashed it off in disgust... and assuming I'm visualizing the right model, here goes.
You will probably have to fashion a key, and yes you are right a sort of fork is what is required. The originals looked like a half washer, with the centre half hole clearing the 'pip' A little file work on a strip of steel of width to fit the slots is in order. Vice, hacksaw, file and patience are required. If you want to hardned the tool after filing to stop it flexing heat it to cherry red and drop it in to water. (conversely if its too hard to file, heat it and let it cool very slowly.)
If its what I think, you (sorry the lamp) will have an aluminium alloy body, probably LM10 or similar (don't worry about that, its aluminium/magnesium with a little silicon to stiffen it. ) The problem is that what was finger tight to squash the gasket when installed is now well and truly corroded in. You may find it worthwhile to WD40 the threads the day before you have to open it.
If it still won't turn, the other option is to centre punch and drill the head off the bolt. The thread will be something common like 2BA or M5 depending on the era, and a normal bolt can be substituted (or even drill right though and re-fit with a bolt and a wing nut outside if the fitting is a special type that must be preserved..), though either grease it or use stainless steel of the problem is not to recur.
In desperation I have welded a conventional bolt onto the top of proprietary fasteners in the past, but its very much kill or cure.
Of course if you can get the screw out then 5 mins with the hacksaw makes it suited to a normal screw driver next time, and grease it before it goes back!.
Hope that is relevant
M.