How do I open VAC Standard LE MkII chassis?

Hello everyone, 

I would like to check what tubes are being used in my VAC Standard LE MkII preamp. I have tried Allen wrenches, Torx, and other small sets gently and nothing is catching to be able to unscrew the screw. Taking a close photo the screw heads say YFS and while they look a little bit like hexagons they also look kind of stripped. 

Any advice for how to unscrew these as I attempt to do this without scratching the top of the unit? 

Thank you so much, 

Antonio