I don't think it will really matter just which type of 'cold blue' you happen to use, the "pen type" I question.....
more importantly it is the "mark" that you make ,somewhere out of the way , out of sight, like under the grips, and how deep you make it....you have to get below the surface enough to get at "base" metal...some of what I recall is that Armaloy, Metalife, and such, they have about a 2 mil build up and some can actually ingrain, into the metal surface.....this "mark" will in NO way hurt the integrity of the gun, and will never been seen and if it does "darken" ( turn blue) you KNOW it is NOT stainless, then by all means, leave it oiled................yes, this could have been done ,way back, January 24th or so, and some 80 posts earlier, quite simple and easy and yes, its done all the time when there is this type of question.Been doing this since the late 1960's.........

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dan
NRA Benefactor,PMA