All I know is that generically they are a GEC Stephenson, which led me to this
https://collection.sciencemuseumgroup.org.uk/documents/aa110072627
Could relate to the 1 Class, or maybe a competitor for the 101 Class (or even for the 111 Class), could just be an outline.
There are drawings of a GEC Stephenson here
https://www.rmweb.co.uk/community/index.php?/topic/152868-steel-works-locos/page/2/
Maybe they were standard, maybe not