All About Pocket Knives

More of a technical question here--can I use a Delrin handle 27 pattern screwdriver backspring in a WWII era TL-29? I have a TL-29 in good shape, except for a broken backspring; yet a pile of '70s #27 Electricians I can use for parts.

They should fit, or very close. Let me go out to the shop and measure some. I have a bunch of parts.

Well, I don't find a single World War II era TL - 29 in my shop, let alone one that's disassembled.
I would say that the part should interchange, as long as you stay with the same brand. Some, like Robson, made a TL - 29 that was about 3 -1/2" but those made by Camillus were 3- 3/4".
Some of the early ones from World War I were 3-3/8", you kind of have to compare apples to apples
But I think as long as you're using a wooden handle World War II TL made buy Camillus and you're using the later handles, I think you will be fine, although you will have to drill holes for the handle pins as most of the later Camillus synthetic handled TL's did not have pins in them, they used pegs to attach the handle to the liner.
If you can find a set of handles that actually used pins to hold the black composite handles on, you may not have to do anything. Otherwise you will at least need to drill holes for pens in the handle.

I hope this is helpful.