The 390-4: Measures in at 4 1/4" x 6 3/4". It contains a normal page ruling, with 1/4" ruling and 160 pages. It is also bound, which is nice for hanging about in your bag!
374: Measures in at 4 5/8" x 7 1/4" with 128 pages of a rather unusual rule...it's called 'Universal'. I would have taken a photo but Ms. Blackberry wasnt being nice! Oh, and the binding is a flexible type of binding.
393: A wirebound one! yes! Measures in at 4 5/8" x 7" and contains 64 pages. Standard 1/4" ruling.
These books are rather fascinating, actually. The paper feels normal with almost a sticky sort of feel, but nothing to be irritated with. And they have TONS of types!
They have a paper type for everyone, and even do custom work. I highly recommend these notebooks, especially if you happen to work in the field or in wet conditions. I used a basic Bic on mine! See below! They truly are waterproof!