Any Swedish Scuba/Skin Divers Here? Found Vintage Poseidon Gear

I bought these 2 sets of diving gear from a friend who found them in a storage unit won at a auction (hey I live in Miami - it should be an easy sale!).
I listed both sets for $350 today & already I am getting messages asking what I want for the Poseidon tank or tank & regulator, so I tried looking it up but couldn't find anything on the tank?
Poseidon is from Sweden (as were the the storage unit's owner because there was a lot of Swedish things in it)
As you can see it's about 1/2 the size of the regular air tank.
and the mouthpieces on the regulator are unusual shaped too
Has anyone seen one like this? Is there a reason people seem to want it so much? (maybe I'm asking too little?)
Even 1 pair of fins are Poseidon - maybe I should sell each set separately?
Thank you for any information!

 

Waiting for approval to join a diving forum - is anyone familiar with the Poseidon tank(1/2 tank?) and/or the regulator/octopus?