Two ideas are mixed together. Mosin gauges are "coin type" - very cheap and easy to make. You can't use "coin type" gauges for rimless cartridges - these chamber gauges will run you more than $50 each. Point is these gauges are profitable to make and sell by creating the impression that head space is a problem - it isn't. There is a scribe mark on the underside of the barrel receiver assembly that should line up if the barrel and receiver are an original pair.
Modified breech blocks and firing pin assemblies for SKSs are a profitable business because they trade on the fear of slam fires (potentially dangerous if you are in the US and have a 20 round mag.) Incidences of slam fire are very rare and are always attributable to something other than bad luck (poor maintenance, broken sear or broken firing pin). If your SKS fires two rounds for one trigger pull you should stop and make whatever repair might be necessary (probably a damaged or miss-apigned sear).