ps2 Version 4 question?

[CubFan2004]

this may be a stupid question, so sorry if that's the case...

but i have a ps2 Version 4 and i was wondering how to tell the difference between a GAP and NO-GAP?

I haven't opened it yet, but i'm having a friend install a modchip for me and i want to find all the appropriate manuals and what not...thanks for any help...