You can still buy slotted steel screws from Paddington hardware,
They are a wonderful old-fashioned store that has been in existence
for a long time and has stock from way back. For example they have
1/2" countersunk bright steel screws in 3G - 10G. They also have the
countersunk semi-cup headed screws that were popular on 50's kitchen
cupboards and could be used on those strap hinges with a little
modification. Which reminds me, you can also get those screw from
Vintage valve amp repro suppliers.