Simplify the following text for a Grade 8 native English-speaking person.
Original: The first experimental batch ( BR numbers 15201-15203 ) were designed by Richard Maunsell of the SR in 1937 and were later classified D3\/12 .
Simplified: These locomotives ( BR numbers 15201-15203 ) were built by the SR in 1937 and were later classified D3\/12 .