Pretty sure the date is ink stamped on it. No hard stampings for date. Hard to find an original one.
I have one, but some nimrod painted it black. The good news is the paint protected it from rust and pitting (which many have) so it is a good candidate for restoration. I have not tried to remove the paint. Was planning to send it to Kirk Connell, but couldn't decide on a date. He does good work at a reasonable price. He did the one on my Z, but I don't remember the cost. Very quick turnaround.
If you don't find a pristine one dated the way you want, send me a pm.
They are being reproduced now. Have no idea how true to original the reproductions are. Also don't know if they will stamp the date of your choice on there. Kirk does.
https://www.ebay.com/itm/Fan-Clutch-...rd!73034!US!-1