When America-on-line, known lto many as AOL, started back in the 1980's they asked for a "screen name" when you joined. Prodigy back then, another online provider, assigned some whacky number for you to log on like R976249765@prodigy. A screen name was something new...
At the time, I was restoring vintage American motorcycles and my own ride was a 1947 Harley Davidson knucklehead. There were flatheads, knuckleheads, panheads, shovelheads etc. These were usually shortened to flatly, knuckle, pan, and shovel.
I could think of nothing else. To this day... if you see knuckle47 ANYWHERE, it's me!