The TSA Secure Flight info you have to submit when you book a flight into US airspace cross-checks your passport info with the SSSS list along with other info the airline provides. The whole process neatly integrates with the airline's reservation system.

I haven't got a SSSS since UA went SHARES but some of the old CO SSSSes were so small that they were easily missed by security staff. They were asked by a few groups whether they could change the size of the SSSS but they said they could not due to system limitations. Some CO stations, particularly in Canada, were handwriting "SSSS" on SSSS BPs so that they wouldn't be missed.