Senor Sea'Scape, thank you for the posi-drive tip. A whole bunch!
This tool/fastener came along after I retired and I was completely unfamiliar (that's a polite way of putting it) of their existance. With the screw one slot is full width like a flat screwdriver slot, the crossways slot looks fit for a phillips but then there is a large pit where the two points of a phillips would cross. The pit looks square but a Robertson will strip it out instantly.
Your answer regarding the posi-drive has really helped me and hopefully other tinkerers who run up against this. Many thanks!