Thanks for looking at it again but I believe the 1st stage is not the device on the back of the 2nd stage. The 1st stage, from what I can tell, is item #7 on page 207 & described on the bottom of page 206 & the middle of page 208. Also there is a diagram of the 1st stage on page 209. The device in question is on page 213 but is the bottom half of the top picture; items 4 & 6 on 213 seem to point to it. Here is a picture of it from the webpage
Strany potápěčské - vše o potápění - Záchranný přístroj ZP-10:
This website has a description of it, which of course, I cant understand. Sorry if I am
but the it just doesnt seem to make sense unless the the 1st stage is split in two with one half at the manifold & half on the back of the 2nd stage. Thanks,