Hi
You have a belt pulley guard Left and Right!
If you look carefully you may find the numbers E151-GE-9 on them.
Page 95 of the Dexta parts list.
I think it was designed to rotate to allow different angles of belt to the pulley.
If you search on EBay you can find flat belt pulleys, there's one up now http://www.ebay.co.uk/itm/VINTAGE-TRACT ... 19db5a852f which has part of the guard on it.
Cheers
Jerry
Jerry Coles
Camerton, Bath, UK
West Highland White Terriers, Dexta's, E27N's and DUKW's