I have used this pinball serial number pinball database for 6 or 7 years and find it an excellent resource.
You can determine with a few detective style points of reference when a machine was exactly made date wise, what number your machine was on the run (237/4000 built) country machine was sent to, and many other factual things about a machine by studying the various stickers under the Playfield, stickers inside the cabinet and stickers from inside and outside the headbox.
Being into classic cars and being an owner of various XA XB XC Hardtops, and Ex Bathurst race Cobras, numbers tracking has always been a great resource.
Also if your pinball machine gets stolen, if you have entered all your serial numbers into the database, you have a leg to stand on to prove that it's your machine etc. Great resource.
