You have a Lineol 4cm scale heavy field howitzer. Here are photos of one that sold on ebay for 178 Euros, but it had 3 composition figures and composition shell cases, the wheeled trailer and spades, and (most importantly) was in it's original 3 color camouflage paint. Yours has either been repainted in silver or has had the original paint stripped first. It is a restoration project...
Hope this helps.