Information from: Bulleid Power - The 'Merchant Navy' Class by A J Fry (Alan Sutton Publishing, 1990):
A large cover, the width of the coal bunker, completely enclosed the vacuum cylinders.
Tender No.3124 (running with 35013) was modified this way in December 1952. Tenders 3122 (35012) and 3121 (35011) were modified in the same manner.
Regards,
Glenn