The GNR A Class 0-4-2 has an intro date of March 1867 with an end of production date of July 1885.
The GNR H Class 2-4-0 has an intro date of May 1867 with an end of production date of May 1885
The GNR sturrock tender used by both has an intro date of January 1860 with an end of production date of July 1873.
The result is that for 12 years starting July 1873 one can purchase a brand new GNR A Class 0-4-2 or GNR H Class 2-4-0 using an already out of production (obsolete) GNR sturrock tender.
The reason I am not fixing this myself with a pull request is I have no idea what the fix is meant to be. It is possible the production dates on the GNR sturrock tender are wrong causing it to end production sooner than it should. It is also possible that the GNR A Class 0-4-2 and GNR H Class 2-4-0 are linked to the wrong tender, using an older tender than it should be.
Thank you for the report - I believe that I have now fixed this.