I'm confused...
The model 69 is a revolver. Why is this in the Semi-Auto Pistols forum? I'll move this to the Smithing section.
Try some different size Allen wrenches (inch, metric...) and see if you have one that will get a bite on the head. I've probably got over 100 wrenches and bits and some are oddball sizes.
You could also try using a torque head driver. If all else fails, clean the screw head with alcohol and epoxy a wrench into place. You'll want to replace the screw when you get it out, so there's little down side.
Another option would be to use a Dremel tool with a disc cutter to create a slot in the head, but you'll probably damage the grips.