the people who made called them "600a" are stupid. The "a" used to mean it had an L2 cache, in the Mendocino days. There were 266 and 300mhz celerons that had NO L2 cache. Then Intel released new celerons with a 128kb ON-DIE cache. They were at 300mhz and 333mhz. To differenciate between the old celey's and the new celeys w/ L2 they put an "a" at the end of the name. So 300mhz celey w/ L2 is called 300a. No L2 it'd be called 300.
But there were no cache'less cely's after 300mhz, so you didn't need to use an "a" in the name anymore. But there are ppl who are slow and un-informed.