I got some new (old) cylinders. One was a small 25/25 cylinder with the key stuck in it. When I disassembled it, I was amazed to see six pins instead of the maybe three I was expecting.
The housing pins were tiny and also very short, except for one tall one, that has a double function as an anti-drill pin. The corresponding key pin is also made of hardened steel. There is one regular housing pin that is longer than the others. You would expect the key to have one cut that is deeper than the others, but that is not the case.
After reassembling, the cylinder worked fine again. Here’s a comparison of the key to this cylinder with one of a regular 6-pin cylinder:
Edit: it is a lock from the manufacturer DOM.
Photos CCBY4.0 Walter @ Toool Blackbag