First of all, you have a great friend !
You have a what many(myself included) consider to be a good quality shotgun of the likes of the "B" guns but does not command the same money as far as resale goes in equivilant as possible grades.
The 605 If I remember correctly was after the Ithaca and Mitsu importing was over and done with and so is marked as an SKB. It has choke tubed barrels ?
I had an Ithaca model 600 12 ga. which was the previous generation and still have one in 20 ga. the next gen was the 685.
Grades of O/U started at the 500(505, 585), 600(605, 685), 700, 800 etc etc
500's were all blued receivers, the 505 and later 500 series grayed receivers
600's early production blued receiver, later 600's and later series were grayed with more ingraving and so on, 800 series came with sideplates.
All use a low profile receiver with trunnions and Greener crossbolt for lockup, barrels crome lined and a black crome finish instead of a traditional bluing.
Tim