Help identify my boat/ history
I am not familiar with the maunfacturer that you mention, General Developmants Co.(Glasgow) Limited, but perhaps I can help with the LOA question. Historically, LOA was measured between the point where line of the stem met the line of the line of the deck to the point where line of the transom met the line of the deck. This was how LOA was measured here and abroad for centuries. LOA did bot include bowsprits, boomkins, anchor rollers, overhanging booms davits,or any of the other items that marketing types started including in LOA about 15 or so years ago. I am not sure if that helps with your puzzle.
If you desribe the boat in more detail maybe I can tell you more about what you have. I assume she is wood. Is she a double ender? Does she have a cabin? A bit more detail will help and I will see what I can dig up.