Ken- That's an Auto-lite 'serviceable' spark-plug. If you look in the old-car magazines (like Model A News, Restorer, and Cars & Parts, you might see references to it. That type of plug has two pieces- you've only got the core, but there's a threaded sleeve with a lock-ring. What a guy would do, is go to his Auto-Lite dealer, and buy a set of four or six 'sleeves' (depending on wether he was driving a Ford or Chevy, hee hee) and a set of five or seven 'cores' (like what you've got). You thread the sleeve into the spark-plug hole, and then drop in the core, and tighten the lock-ring, which squeezed against the core. This deraingement allowed you to do three things: 1) Clean the plug reely easily (if you had a Ford), 2) blow the raw gas out the cylinders and re-gap the plug (if you had a Chevy), or replace a broken plug (if you were a Dodge-drivin' Scotsman).
As testament to the validity of the design concept, this method is clearly extinct today, partially because the plug wouldn't hold up against modern compression ratios (gee, I think Dad's 29fordA developed 5.5:1), and since the 'heat path' from plug tip to the cooled head is impeded by a questionable joint, clearly wouldn't stay cool enough to prevent detonation.
But if you'd like to experience the 2-piece spark plug for yourself, replace your average Small Block Chevy's AC-Delco R42T with a so-called 'equivalent' Champion, and take it for a good-hard ride. When the collar-crimp fails, it'll spit the core right out, and it'll be just-like-old-times.
(yes, I had this exact scenario happen to me while cruising around in the boat last July 23rd. Lucky for me, a guy came cruising by that'd just changed his Delco plugs... so I bought one used MR42T (marine version is corrosion-resistant, hence the M), and went on my merry way.
Needless to say, there was a whole lot of fancy spark-plug experimentation between 1928 and 1950, and with all the 'drastic improvements' touted by 'breakthrough designs', the ordinary spark-plug is still used as original equipment. Alas, there's quite a few people who pay two and three times the price for a 'gimmick' spark-plug, and swear it improves performance. My favorite was seeing the ads "Fires in OIL!" (uh... Duhhh... it's a conductor!)