I'm honestly not sure, but the Drain Cock most likely have a 1/4" MPT thread, which will actually measure 1/2" outside diameter (from outside of one thread to the other). You always subtract 1/4" off of the O.D. (outside diameter) to obtain the "MPT" size.
So, 1/4" MPT is by far the most common size.
- MTR
After some research I found the exact model# match...this is a C0300809 Powermate compressor. I've attached the .PDF parts breakdown for your reference but it looks like the Drain Cock part# is DU25-100 (MTR's part#) which sells for about $4.99 or so.
Some models used a Remote Drain Valve (which you'll notice on the .PDF) which is part# 136-0072 but it's not cheap at $35 or so. This type of drain valve is foot-operated.
