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In this response to the APU location on an L-1011, in this image, there appears to be a large piston or leg extended downward along the centerline from the aft wall:

enter image description here
Modified: By Reedy (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Question is, what is that and what is its purpose? Silly guesses: A giant locking pin? A shock absorber?

Definitely does not seem like an effective tail-strike device. Also does not seem to be identified in the cutaway

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It's a retractable tail bumper. It's not available on the shorter -500:

enter image description here
The aft lower fuselage. A retractable tail bumper in the left side of the photo is on all the L-1011s except for the shorter Dash 500s.
Source: Upton, Jim. Lockheed L-1011 Tristar. 2001.

On a closer look the reference does not mention its purpose. But the flight manual mentions a "tailskid" being powered by Hydraulic System A, and then under the landing gear section confirms its purpose by saying, "On takeoff or landing, tailskid contact will occur when pitch attitude exceeds approximately 12.5°." (Page attached; emphasis added.) So, it's an anti-tail strike device.

(Given its shape I initially erroneously thought it similar to the ATR 72's "pogo stick".)

Also mentioned in an NTSB hearing c. mid-70s, the FAA asked that the L-1011 tailskids be painted to make it easier to spot a contact:

According to Mr. Tamalis, the manual is more definitive than the original manual with respect to landing techniques and, if the F.A.A. feels a revision is in order, it will ask the company to make such a revision. In this case, the revision was initiated by Eastern Airlines after the incidents and approved by the F.A.A. as a more definitive statement of procedure. The manual is more definitive in that it speaks to body angles for the first time, as well as reference speeds for landing. Also, Mr. Tamalis testified that there was a series of tailskids striking the ground with the L-1011s and, therefore, the F.A.A. asked the carriers to paint the tailskids so occurrences could be observed more quickly during a course of a walk-around.

Related: What is the indicated device under the tail of this B-29?

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    $\begingroup$ Wow, thanks for such a detailed answer! Am not an engineer, but I stand by my thought, "does not seem like an effective tail-strike device"; I would have thought en extensible convex skid would be a better design, more like what's seen on the B-29 you reference. The torque on a vertical pin like that must be huge if for some reason it can't "skid", like hitting a rut. I imagine it could inflict significant damage after a contact incident (hence the paint requirement)! $\endgroup$
    – Ian W
    Commented May 18, 2022 at 20:09
  • $\begingroup$ @IanW: Yes its shape is strange! Then again it has a shock strut / damper. The paint bit proves the purpose, because a tilted L-1011 at the ramp doesn't need paint to notice :D L-1011 was overengineered in some areas, and it seems this is one of them. A book that is not available for viewing hints at the complexity and a special alloy perhaps (on my to-do list to try and look into but no promises). $\endgroup$
    – user14897
    Commented May 18, 2022 at 20:29
  • $\begingroup$ @IanW - At least the bottom surface of the cap on the bottom of the strut is convex. Seems like it ought to work ok on a paved runway. $\endgroup$
    – quiet flyer
    Commented May 20, 2022 at 16:52

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