It's a reproduction of a 1960's poster that introduced the newly modernised Metropolitan Line 50 years ago, with the "new" extension. It's £9.95 and available online here.
And for those who are old enough, you might remember the iconic Metropolitan line trains with their high back seats and commodious luggage racks. London Transport Museum have rescued a limited number of these original 1960's luggage racks from decommissioned Metropolitan Line trains. You can buy them online from £150 or at the Museum’s Covent Garden shop from March 2012.