Hmm, the
problem is that I don't know what model that is. On the box
there is nothing that might help to identify the model type, the only
thing I know is that it was a little one (the smallest and narrowest)
model with added storage expansion (which makes it twice as wide).
It should say somewhere on it, though not likely in a big flashing sign.
It sounds like you have something like a 9406-400
If it's a small model, I'd expect it to be a 9402. Intermediate sized
machines were 9404.
If it's an older CISC architecture machine, the model suffix might have
a letter and two digits; the one I had was a 9406-B45; the B series
was pretty old. Later CISC and most (all?) of the RISC models have a
three digit numeric suffix.
It's definitely true that IBM didn't make it easy to identify AS/400
systems or their options.