Shirley,
The top hat cams are called that because from the edge view they look like little flat brimmed hats. There is a raised part in the center and a mouse hole shaped hole for the locator peg on the flat part. The other Singer cams are flat.
The built in cam stack has a big bunch of stitches. And by varying the controls of the machine you can alter them to your tastes.
Sadly I have a bunch of good pics on my other computer drive, the one that died, that I'd post if I could.
The insides of our 401 is steel, steel, steel. Not one tiny bit of plastic in there. Pull the top off of that machine and look at it. The cam stack is right below the trap door for the removable cams. Directly under it, you can't see it without a mirror is the steel driver gear. The controls can be jammed up if you turn one nob without resetting the other nob. Set one, make sure the nob has latched, then set the other one.
When you lift the trap door there is a chart, or should be, showing all the built in patterns and where to set the nobs to get them.
Plus in the manual I downloaded and printed (a large .pdf file) there is extensive instructions on how to set the controls with and without the cams. That too is on my dead PC and I'll try to find it and post it for you.
The 401 is one machine that benefits greatly from Tri-Flow. Lots of things to oil and grease. You especially need to oil and free up the parts that move up and down on the two vertical shafts just to the left of the cam stack. If those don't move free, you'll have fits getting the cams, built in or removable, to produce the patterns.
When we got our machine the cam stack was not jammed up, but the actuators on the vertical shafts almost were, and someone had put what looked like black chassis grease on the cams. Ugh, what a mess. That stuff had solidified and I had to scrape it off one little bit at a time. Use Tri-Flow grease here.
I've got an errand to run in a few minutes but when I get back I'll pop the top off our 401 and take some pics for you if you want.
You got a good machine. Once you get it cleaned and lubed I'll bet you'll smile as you use it.
Joe