The followed cars were renumber for Parcels service:
50725 became 55948
50726 " 55938
51573 " 55934
51576 " 55935
51579 " 55936
51581 " 55937
51582 " 55939
51585 " 55940

Other vehicles saw parcels use, but were not renumbered, such as 53703/53704.

Many more shots of parcels use, more can be found in their own gallery.
Downgraded to parcels work, 51580/51574 wait at Chester in the rain on the 25th October 1986. Paul Moxon

51582 is seen in the company of a 114, and paired with 51573. Leeds, 27th June 1986. Malcolm Clements

51573 at Leeds, 27th June 1986, with Express Parcels branding. Malcolm Clements

51785 is paired with Class 116 53072, both carrying a Red Star logo. Leeds 27th June 1986. Malcolm Clements