Now that you mention that I know it's slightly bigger because I bought an A2 portfolio to store my unframed lithos and they wouldn't fit, I ended up buying an Itoya 24x18 portfolio from
nipperkipper who imported 2 from the USA and kindly sold me the spare at cost.
I'd REALLY hate to cut any of my lithos down, even though I
know I could do a good job after working at a print finishers for a year........I'll just have to suck it up and pay a premium.