Thanks for the comment. Anyway I still did not find the serial number on the frame. Maybe someone might help me sending a photo of where it is supposed to be. Maybe they never put in the number on this plane?
I can't speak for the Model I airframe but on the Series 5, 6 & 7 the serial number is stamped on the small triangular shaped metal tab where the seat belts bolt onto just behind the pilot and passengers head.
In the picture you can see these two triangular shaped tabs with a hole drilled in them for the seat belt connection.
The number is stamped into the tab. Powder coat and paint may partially fill the stamped numbers so they may be somewhat hard to see with out very close examination and maybe even some sanding could be required to clearly make out the numbers.
If the Model I used the same location then maybe this will give you a specific place to look.