http://www.motorcycl.../_/R-EPI-133589
The British Motorcycle Racing Club (Bemsee) has produced a short booklet clearly showing what chassis and engine numbers should look like, along with their forged equivalents. There are also clear descriptions of how ringing and cloning works
I used to play spot the iffy bike at club meetings years ago. There were always a lot Triumph engines that had been attacked by a grinder.