The 50 is booked for enclosed half top operation all year round using 1401-1403 although we usually use closed tops on the last two return trips from Swanage at 1725/1825 (ex Bournemouth 1838/1938).
The first convertible roof comes off around May when the 50 timetable steps up slightly following the the end of term at Bournemouth Uni. The other convertibles then go open top when the Purbeck School runs end in July in time for the 50 to go into it's half hourly summer frequency.
Although this summer may be different with the new half tops expected in May.