Original: When the Class 97 series for self-propelled locomotives was introduced, some locomotives in this series were renumbered to Class 97, leaving the 968xxx numbers just for locomotives that were no longer self-propelled.
Simplified: Before the introduction of the Total Operations Processing System, Class 97 was issued to self-propelled locomotives in departmental (non-revenue earning) use, British Rail had such locomotives numbered in a variety of series, together with locomotives that were no longer self-propelled.