Version of PS2?

[fenston1]

Can anyone tell me an easy way of knowing what version of PS2 you have? (So far I've been lucky choosing my versions for the Mod Chips I order) but with the last "solderless mod" it claims that a model number scph-3000x R is what to look for, but then asks for an extra "2 screw slot" within the drive bay... I've come across one with the right Model Number, but no screw slot... Which should I rely on, and if the screw slot isn't there, is there a way of knowing which model it is?) Is there no rhyme or reason for confusion's sake? (I've started modding my friend's PS2's, and I'd like some sort of "stone answer" when ordering the appropriate mod chip.) Thanks!