I know everyone has correctly identified the "L" seat-post but here is something that might interest you all when people still rode bikes or tandems fitted with this type of pillar. Back in the late 50's when I first started riding it was common practice amongst the older riders to keep a few non-safety matches (the sort that will strike on anything Swan Vestas etc.)dry whilst riding in wet weather by inserting the matches into two corks and pushing them in to either end of the saddle pillar to keep them bone dry. This ensured that when we stopped for a "Brew-up or drum-up" slang for making tea on a Primus stove or a "Fag" (smoke) we always had some means of lighting them with dry matches. Most of these old-timers still smoked way back then.
Thats all for now. Keep those wheels spinning, in your memories if not still on the road. Be lucky Mick Butler Huntingdon UK.