Looks very much like a 37mm (2lb) ,falling block,Nordenfelt gun. This single shot gun was used on small anti torpedo boats. I think the 33kg refers to the weight of the gun (approx. 72lbs).
Favored by the Spanish Navy, many of them where captured by the U.S. Navy in Cuba. There are variations with a shoulder stock. I saw one so equipt at a Maritime museum in Washington state.
Vlad and Scott, this is Hotchkiss 37 mm L/20 rapid fire gun used by the Spanish Navy. Bannerman had these for sale at a “bargain price” that included 500 assorted loaded and blank rounds.