Does it work? Have you tried it yet? Show us the insides if you feel like it, i wonder if it's a reed valve..

here's a page with a picture of one with an exaust collector fitted.
http://www.pebotec.de/peter/motoren/cipolla/cipolla.htm

In fact this guys site has lots of pictures of 1/2a and other small engines from all over the world;

http://www.pebotec.de/peter/motoren/motoren.htm