I've found some new photos. It's quite difficult to search through Korean language but somehow I managed to get these pictures below.
That's not everything, I also know the answer what's written on the sign.
I asked Dong-Woo Kang from South Korea(our forum member, N96HBK) and he explained the writing is: 주체포(juchepo/juche-po). It is the official North Korean name of this artillery. "Juche" part is probably a reference to the NK national ideology created by Kim Il-sung.(
https://en.wikipedia.org/wiki/Juche). "Po" part means cannon/artillery. It makes sense. An artillery that fires DPRK's ideology shells... Sounds dumb but in North Korea everything is possible.