I conferred with my fastener buddy about your fastener and here are some of the conclusions reached.
He has seen fasteners with your approximate dimensions but they had thread forming threads.
The bevel at 120 degrees is unusual in respect that the normal bevels are 82, 90, and 100 degrees. The 120 degree bevel is common on flush rivets for thin skin applications.
We tried to use the backdoor approach by contacting manufacturers of 120 degree countersinks. No luck, they professed no idea of the end use and didn't part with any names of purchasers.
A local box builder, no longer builds, couldn't tell him any of their suppliers even though they were sure they used this size. They were no old hands around.
His suppliers only offered the 82, 90, and 100 degree head as standard. Both had a a quantity 1000 for non-stock items. This might be of help if you find someone who makes them.
Don't give up, strange things have happened on the way to the Forum.