A Regelbau, R633, housing the M-19 Maschinengranatenwerfer, automatic 5cm mortar. This one is from Fjell Festning, Hordaland, Norway.
<a href="//commons.wikimedia.org/wiki/User:SinWin" title="User:SinWin">SinWin</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>