Weird concave coins

Hello again,

A friend of mine, who sell coins, have this set of weird "coins", he asked me if I know anything about them, I have no clue and told him I would post here, where the best coin IDs of the world dwell.

Have you ever seen such coins?
I know byzantine coins are concave, but those looks more crude and homemade...

Thank you for any comment.

They are what you call in Germany Hohlpfennig.
You might find some links explaining about them on google https://www.google.de/search?q=hohlpfennig&ie=utf-8&oe=utf-8&client=firefox-b-ab&gfe_rd=cr&ei=10ZDWJPIGIne8gfv7avwDQ

They where hammer struck with something looking like this http://blog.muenzversand.at/2015/03/28/friesacher-eriacensis-pfennig-pragestempel-bei-rauch-auktion-97/

Other simillar types but with a slightly different shapes are called Schüsselpfennig, more here https://www.google.de/search?client=firefox-b-ab&q=sch%C3%BCsselpfennig&oq=sch%C3%BCsselpfennig&gs_l=serp.3..0j0i30k1l3j0i5i30k1.9563.9563.0.9730.1.1.0.0.0.0.95.95.1.1.0....0...1c.1.64.serp..0.1.94.6802Gg4AENA

Thank you Apuking!
I have never seen anything like that, that is a new level of knowledge! Very very thanks!