Leg, he is looking for a longer barrel for trap shooting .... maybe 26", 28" 30" .... A blued barrel in that length ..... would that be a Police barrel?
And once he found a barrel somewhere ... how will a newbie figure out if he is facing a parkerized Police barrel or an Express matte blued barrel?
I guess the answer is simple .... whatever he finds in that length ... 26", 28" 30" .... if it is a polished shiny blue ... it is likely a Wingmaster barrel. If it is a matte finish .... it is likely an Express barrel. Given that it is an original Remington barrel and not some third party barrel like Hastings, Carlson, etc ....
Anyhow, any 870 barrel in that length category should fit on his 870 Police receiver!