Gromit and all,
The reason the fork and upper block stay aligned is that when the pendulum is being spun around, the fork can only move as far as the anchor pin will let it. That's about 30 degrees, which is NOT beyond the elastic limit of the suspension spring material between the fork and upper block. From actual test, you need to twist the fork around at least 60 degrees to exceed the elastic limit and produce a "slight" twist in the spring segment between the fork and upper block.
When the pendulum quits spinning, since the elastic limit has not been exceeded the fork will return to its normal position and (most of the time) the clock will continue to run even though the suspension spring below the fork has been turned into a corkscrew. As I mentioned earlier, I have done this on purpose to check out what really happens.
Regarding twisted springs being original equipment, I am highly skeptical of that having ever happened. I have a large collection of original Schatz, Kundo, Kern, and other maker post-WW2 catalogs and technical material, and no mention is made of this "feature" in any of them. Charles Terwilliger made no comment whatever about this in the Horolovar 400-Day Clock Repair Guide, and he saw a LOT more 400-Day clocks "as shipped" from the makers than I ever will. Not only that, since he was "the" major producer of suspension springs in the 50's and onward, I don't think there is any way he would not have noticed such a feature and at least commented on it. Further, technical literature from "way back" strongly emphasizes the need to have a perfectly flat suspension spring to be able to properly regulate the clock.
I also am positive about the following:
1) Horolovar has never supplied corkscrew suspension springs. In an earlier post I mentioned that I never say never, but this is one time I can do that because I personally know the owner and have spoken with him on this subject.
2) I have opened "many" new old stock spare suspension spring packets from Kundo, Schatz, Kern, Koma, Kaiser, Reiner, Würthner, Haller, Nisshindo, and others and have yet to find a corkscrew original spring.
So . . unless and until someone shows up with an unopened original spare suspension spring packet and can show me X-rays that it is twisted, I'm going to say 'tain't so.
John Hubby
P.S.: In addition to "K und O" for Kieninger and Obergfell, there is also "KO MA" for Konrad Mauch and "HE CO" for Henry Coehler. Probably a few otheres as well.