There are some galley carts are fully interchangeable between all UA mainline aircraft. And there are some that aren't.

The "CA777" refers to the type of cart, not the type of aircraft.

Galley carts (made by the same manufacturer) are all standard width, but some are longer than others. And, some have different interior shelving configurations.

UA's aircraft galleys are supplied by the same manufacturer so width-wise the equipment is all the same. There are just different depths to the carts based on the location they are to be stored in the galley.

I'm sure a FA can elaborate further.