Can someone knowledgeable on the subject point me to the earliest few locking visors or visors with catches? Artistic and/or documentary evidence would be deeply appreciated. I'm assuming this happens sometime around 1400, but I'm prepared to be wrong...

Will McLean says that the first examples are mid 15th century, and that it was not a usual feature until later. Being able to quickly open and close your visor had advantages. The oldest helmets with latches on the visor at Churberg are Nurnberg work c. 1510 eg. photo no. 160 in Hans Prunner's book.

When I flip through Edge and Paddock, the sixteenth-century close helmets usually have a latch, like Prince Wladislas' armour of 1510. I don't see anything earlier.

http://willscommonplacebook.blogspot.com/2012...tacks.html

Never really paid attention to that feature, but I did a fast scroll through of some helm images I have and found several sallets with latches from the 1480's and 90's.