I am guessing that you are from the U. K. I have visited several times and would love to go back. There are several tools in my collection that I brought back as souvenirs. You are luck to live in an area that I have found to be rich with wonderful woodworking tools and traditions.
I had the same issue with photo file size when I started posting but I found that you can resize them using Windows paint which is commonly found in Windows accessories on most computers. We don’t get to see many Salmens planes in the U.S. so my comments are based on what little I have been able to find online. Salmens did produce planes that were painted red from the factory and that could easily be what you have. However, it might be that that a previous owner just liked the color red and repainted it. I know of one popular online woodworker that has repainted many of his restored tools a unique shade of blue. The “CO 457” marking would make me believe that it is a later (as in 1960’s or later) issue plane. Older lines of planes had relatively simple designations and modern management has introduced more complicated stock numbers for tracking. Stanley had several different frog/base configurations and it might be that yours was copied from one of the less common designs.
Don’t get too caught up in the plane identification and whether all of the plane parts are original unless you are concerned about collectability. Many of the Stanley Bailey pattern plane clones have somewhat interchangeable parts and many of them have had parts from different manufacturers. That might sound strange, but consider how many planes have had the blades replaced with aftermarket blades. It would make sense that an owner with several planes with issues would “rob” one for parts that would make another one useable. The big thing for most users is whether the plane functions well.
A #4 is traditionally used as a smoothing plane but keep in mind that the designation of “smoothing plane” is a function of how the plane is set up, tuned, and used as well the length or manufacturers designation. It isn’t unusual to see planes in the # 3 through #6 being used as smoothers even though the #5 and #6 are less common for that purpose due to length. #1 and #2 planes are generally “smoothing ” planes but not many have them and most that do don’t use them. I would tune it based on need and the best function of the tool. At the absolute worst you will probably have a good scrub plane.