I asked my friend who studied maths at Trinity College Cambridge, worked as head of maths at a school and now works giving training to maths teachers.
Even he wasn't too sure but did have a couple of ideas, here is his reply to me...
"Good question! I think like the two triangles which are included it is about being able to produce diagrams which are consistent and accurate, but to be honest I can't think of a good mathematical reason why you would need the alphabet or numbers on a stencil. Perhaps young children could learn to write letters or numbers using a stencil?"
...I'm not sure it is designed for young children with the set also containing a compass and pencil sharpener. I'm sure most people wanting a maths set like this are already capable of writing letters and numbers.
Producing consistent and accurate diagrams send more likely, although even with all his mathematical experience my friend can't think of a good mathematical reason to need the alphabet or numbers on a stencil.