Crosman 600 help

Hello! So this is my late grandfather's crosman 600. I'm completely new to air guns, so don't forgive any ignorance on my part. I heard that there are 3 generations of the 600, how can I tell what version I have? The end cap that screws on the front is nowhere to be found. Not really sure what else to ask....if you need more info I'll try my best to provide it.