Geeze,......and I thought we only had to worry about rust on our steel French C&V bikes....now we also have to watch out for termites too??

Chombi

That's interesting, Scooper. As far as Chas' split sleeve theory, I'll have to look. As far as the straight gage theory, very interesting too. The steerer on the Moto I'm working on has "Reynolds 531" stamped right on it, but still may be straight gage tubing I suppose.
BTW, the wooden plug in this steerer was quite loose, but may have been snug when installed. Wood shrinks, after all.

Looks like I'll start a new cottage industry ...custom hard wood fork plugs. Think they'll sell?

I had a bike that I wanted to re-finish, and being super anal I removed the wood and it was a huge job, it was like petrified and because of the hardness and the "rifling" of the Columbus steerer I had to drill and chisel it out.

Thanks for posting the interesting link: some good arguments and I'm half-way to believing in this as best practice (for a specific and limited application).
The thing I kept thinking is: how many "All 531" frames used something other than the 531 steerer (which AFAIK WAS supplied as part of a complete tube set)?
Whether cheap split and rolled mild steel or a "butted Nervor" steerer, you can't call that "tout 531" in my book.

My early 80s CFX-10 has one. I guess I never really thought about its purpose, but it hasn't caused any problems (no interior rust) and looks pretty cool.

Yep; a butted steerer was part of the 531 tubeset.

My PX10 still has the wooden plug in the steer tube. It's short, loose and would do absolutely nothing to support anything in a crash. The only explanation for it that makes sense to me is that it keeps water from the tire out of the steerer.

My 8 top-level Peugeots (PX/PY, 1962 - early 1978) all have this wooden dowel inserted into the steerer tube. It's tight fit on all of them, one has to drive this thingy out with a hammer. The plug is 5cm / 2 inch long and made of hard wood.
3 tubes Reynolds Peugeots (PR) and gaspipe PAs don't have such plug (same fork on PR and PA, as far as I know).
To me it seems to be more of a racer's accessory.

PX and PY do not have a split sleeve brazed into the steerer tube's end (-> # 24 & 31 / very interesting nonetheless!).

In my opinion this wooden dowel must be a safety device.

On my early 60s PX10s I also found a wooden plug inserted halfway into the seattube. My best guess here is that it's to keep away water and grime from the BB internals. None of these bikes had a sleeve around axle and bearings when I dismantled the BB.
Maybe these plastic sleeves were introduced around the mid 60s - just a guess - I never found any information on this.

(Fortunately no rust damage to forks and seattubes)

Obviously, it is there to prevent the jagged pieces of your broken stem from falling out and causing a blow-out at high speed.

(Once again, it goes to show how the French were thinking about your safety, whereas the Brits and Japanese would have you ride plugless, and at your own peril.)

Yes, you have to drive a wooden stake through the heart of these French Death Forks (garlic obviously being part of the problem)!

https://tandem.noir.pagesperso-orange...s/tandem-f.htm
This French blogger points out that the plugs were made from cornel wood. For serious restorations.