I have a bunch of Southco fasteners in a range of sizes from 120 to 340 with description of SOU-28-xxx-16 where xxx is the size.
They are flush 1/4 turn, phillips recessed steel with zinc plate, chromate plus sealer according to the Southco website description and looked good on the twin. I've replaced them with screws.
In the quantities I bought the unit prices were high and shipping was from multiple manufacturers as no one source had all 9 sizes. They are currently selling for $1.00-4.99 each at low quantities so I think $2 each would be fair plus whatever shipping is involved. It's a money loser for me but better to be put to some use rather than send to the landfill.
-----New -----Gently used with ring washer
120 15
140 20
160 20 -----1
180 17 -----6
200 5 -----8
220 4 -----4
240
260 3 -----6
280 3 -----1
300 5
320
340 -----1
Email with what you'd like.