1st means First Class mail. The stamp, one of a series of 6 of steam locomotives issued in 2010, featured the "Evening Star, the last of the British Rail Class 9F locomotive is shown here at Midsomer Norton in 1962. The 9F was designed to pull heavy freight trains. But a few, like Evening Star, were used on passenger services, particularly on the steeply graded Somerset and Dorset line."
See the full set here.