Pneumatic questions, sorry!

[Max]

Ok- The nut on my (2nd hand) Trevor regulator deffinatly screws onto my Co2 fire extinguisher (quite an old one). The thread on the extinguisher and on off valve look pretty simular but from what woody said there is a subtle difference. I think my on/off valve could screw into my regulator but the thread does start early enough (look at the picture)
[attachment=0:3b5etuuv]on off valve.JPG[/attachment:3b5etuuv]
I thought i could file the end of the on/off valve down with a grinder/file so the thread starts instantly- does this sound wise or will it mean i cant use the Valve as it has been modified?