Imperial Pen Knife: What is This Type of Blade Latch Called?

[wlwhittier]

Carbon steel blade, liners & spring...perhaps stainless sheet-metal scales, end crimped.
At the hinge end there is a wee roller that latches the blade closed.
This knife must have been inexpensive in its day (when, roughly?).
I would like to describe it accurately in my listing and don't know what the latch feature is properly called.
Any help with approximate age will also be much appreciated. Thanks for lookin'!

 

[Arathol]

Thats an Imperial trick knife...from the markings third quarter of the 20th century roughly.....

 

[wlwhittier]

Thats an Imperial trick knife...from the markings third quarter of the 20th century roughly.....

Thank you very much!