Let's see, the 550a, was made by Mossberg predominantly (some were Nobles according to sources) and they will rattle especially after they are broken in, as long as it doesn't bind when pumping or the bolt doesn't rattle when closed I wouldn't worry about it too much.
On the mauser , Argentina used a 7.65x53 , a 7.65x54 and I believe another round similarly sized, you'll have to determine which mauser action it's built on to guesstimate age and therefore caliber. your best bet would be to go to www.cruffler.com i think those boys can help you out.
oh and the second number is the case length but then there's the "R" after that sometimes that means rimmed..
but I only know shotguns......
On the mauser , Argentina used a 7.65x53 , a 7.65x54 and I believe another round similarly sized, you'll have to determine which mauser action it's built on to guesstimate age and therefore caliber. your best bet would be to go to www.cruffler.com i think those boys can help you out.
oh and the second number is the case length but then there's the "R" after that sometimes that means rimmed..
but I only know shotguns......