I think Martin Gubbins was probably correct when he wrote:
A well known oddity of York hallmarks is the sterling mark which is sometimes shows the lion facing right in the period 1798 to 1808 and perhaps in later years too. Except for the hallmarks of Newcastle in the 1720's and very occasionally in the next two decades, this anomaly is peculiar to York and is therefore an immediate form of recognition . There appears to be no significance in this right-facing lion and it is likely to be no more than a mistake by the engraver in employing a punch mark, rather than another punch, for his model.
Source: York Assay Office & Silversmiths 1776-1858 - Martin Gubbins - 1983