Help with constellation reference

Guy0783

Good afternoon, I picked up a couple constellations this morning. Both need service and have mottled dials from moisture, but one needs a caseback and I'm not familiar with the reference. I know you all can help.

Serial is 15M range on a 505 movement which puts it around 1956 but it's in a straight lug case. It may be an incorrect upper case as we also found a loose 2852 caseback.

What do you think? I'd like to get it correct. Thanks!