Many of your questions answered here.
http://www.gunfaqs.org/P7FAQ/ Read section VII; "refurbs" for your particular history.
Your pistol looks a bit like this one except for the Nds in the oval I think.
The eagle in the square with BWB is a marking for the military proof house.
BMI was the unit this pistol was issued to. Ministry of Interior IIRC. That makes it a used turn-in.
PW Arms is one of the larger importers of surplus P7s.
One of my P7s is marked as yours... same date, 6218X serial number and I have another with the same marking but a later date, 1995. Indicating these P7s were used by the BMI for at least that long (1983 to 1995). When exactly they were retired from service and imported, I don't know.