Whiteoaks,these were from the '60's.As Bjorn said,they were slotted for a coin (quarter).The slot is .12" wide and not flat bottomed but dips deepest in the center.
The head is .5" wide and slightly domed.The rim is knurled,app. 35 lines per inch.These appear to have a blued finish.I have seen older ones that appeared to be gray parkerized.
The total length,including head,is .567".The threaded portion is .44".The bolts have 1/4-20 threads.I believe this would have been the design from the mid-50's,probably into the '70's.