These two Schrade knives have a locking screwdriver that unlocks by pressing down on the spine of the knife blade. Newer versions had a liner lock (like the Klein knives above).
Thank you, guys. TsarBomba, you should see the locking mechanism (if there is one) inside the well. If you post a photo like this one then me or another forum member will be able to identify it.
Well I finally pulled out my linesman knives and took a closer look at the Schrade. Though it speaks to a possible loss of cognitive facilities in my old age

it does appear I misremembered a detail about my Schrade after all. First, the outside:
Its original owner seemed to have a thing for that strange slanted-line "rune" on both scales. This one is nowhere near as pristine as yours, obviously, but I like to think it had a good, proper, honest working life before it got to me, and the marks on the scales and bolster are "character lines".

It is still plenty usable, so I opened the blades to check the guts.
Well, what do you know, under a flashlight that looks like a slot and platform that could be part of an integral lock! I closed up the master spear and went to fold the driver blade into the handle, and what do you know, resistance! Well, you know how those linesman's knives have beastly thick and powerful springs... Gave it a little more juice, yet the driver's going nowhere. Push down on the master and suddenly ol' Bob's yer uncle! The driver blade swings closed with a minimum of effort.
So I'll revise my earlier comment, I have a
working Schrade TL-29! Be it ever so humble, it's a nice little piece of history. Here it is with my 4-line Camillus version -- doesn't the shield just class that Schrade right up?
Thanks for sharing yours, Jake!