Looks like a very nice one. The full fairing version police version appears to be 1988-2001. The semi-fairing police version is 1985-1989?
Anyway, there are 4 police manuals out there and they vary in years. 850 T5 Polizia (1985-1989).pdf, 850 T5 Polizia (1988-1993).pdf, 850 T5 Polizia (1994-1994).pdf, 850 T5 Polizia (1995-2001).pdf - they likely have some changes, but they mostly look the same to me.
I know for sure that some of them had difference swingarms. The older ones had straight swingarms, the later ones had the curved ones with the indentation to accommodate a wider tires (same as Le Mans 1000 swingarm), then there was something about the civilian but I don't recall, 850 T5 Civile (1985-1988).pdf. They were all medium length however.
These guys here restore them and bring police models back making them street legal. Maybe that is where that one came from, hard to tell. Anyway, these guys may be able to help you with details, but they are a little slow to respond and English is not their main language.
http://web.acr-ricambi.it.spazioweb.it/moto-guzziGood luck.