Rolco was a Wilsdorf Brand, much like Tudor is.
This is probably 9ct Gold, and will most likely be hallmarked for gold from a UK Assay Office (they used a few different ones - Birmingham, Glasgow etc). The Only Rolex signing is likely to be the RWC Ltd (Rolex Watch Company Ltd) on the inside of the case.
It will be a bought in movement, possibly slightly tweeked to be a Rolco one (rather than immediately identifiable as a third party one), and most likely signed Rolco on the ratchet wheel.
Nice little thing.
Hirsch strap
ps - 1920s I think, the brand wasn't used for very long