Your friends were some dam fine mechanics. Not many know that trick. The way I knurled the bullet in the picture was to lay the boolit on a file that was on a bench then take another identical file and roll the boolit between them. With a little practice it works very well. For those who want to hold grease on a 38 or any other long bearing surface slug (like a wad cutter) without grease grooves, this method may be just the ticket for you.
Fyi There is not much difference between knurling and canneluring a boolit. Knurling is more a term for making the checker pattern on a lathe though. Most people think of canneluring when making a pattern of cuts around a boolit or more likely a jacketed bullet. Cannelure machines are also used for making cannelure patterns on cases. They pretty much work the same though. By pressure they cut or mash dents into the bullet or case by rolling it between some type of forming tool.
Here is a link to a video I made about rolling a bullet between two files. I did a very poor job in the video but you get the idea here: https://www.dropbox.com/s/o8j2s3z0lu...Crimp.MP4?dl=0