For those who dont know, pinball machine serial numbers for all of your machines can be kept on the IPDB site and recorded.

example below of front serial page

http://www.ipsnd.net/View.aspx?id=4443

then page with the date you submiited your serial number, which proves u have had the machine a while etc. (Note - clicking on your serial number from the above page link gets u to the next page below).

http://www.ipsnd.net/details.aspx?id=17221

That way, its a permanent record so there is no argument if u happen to find a stolen machine, and it is also something that both the Police and Insurance companies could also use as proof of when the game was registered etc and under what name etc.

* I hope you get more of your machines back. Thieves are bottomfeeders like someone else has rightly said.