I have here a 1956 Omega ref. # PX6270. This is a U.S. cased watch with no model designation (DeVille, Seamaster, etc.). Would this particular piece have a model name, or is it perhaps an "entry-level" model?

 

Well it’s the exception to the rule about GF and Plated watched having SS backs.

It’s a US market case made by Star watch. What’s the movement inside?

 

No, lots of Omegas don't have a particular line which they do or don't belong to.

 

Is the caseback gasketed?
Is the crown tube water-resistant?

I consider water-resistance as an indication of a Seamaster. When you have a non-Swiss model, who know's? Who cares?
If it's non-Swiss, collectability is so condition oriented and the value is bound to a lower range than the Swiss models.

 

not every model has a line or a "collection" they belong to, but you can referr to them with an indication based on their main characteristic. For example "solotempo with seamaster style"

 

Caliber 420.

 

There is a gasket on the inside of the back of the case, and the crown tube appears to be o-ringed as well.

 

Price would be inline to similar cal 420 watches.

 

These cases have a very thin gold plated coat that worn off easily at the edges.
I would save the money to buy better ones.