Whilst browsing the http://www.railcar.co.uk website I found reference to:
51899 59744 59764 51900 being allocated 'new' to 1E Bletchley 17th December 1960
51897 59743 59763 51898 being allocated 'new' to 1E Bletchley January 1961 (Stabled at Chaddesden sidings - 12th November 1960 pending allocation)
The above sets remained at Bletchley for 6 months before being transferred to Marylebone.
I wonder what services these units covered; possibly trialled on local services to/from Euston?