George,
If I remember correctly, before there were any compounds, AMO stands for Archery Manufacturers Organization, and when you purchased a recurve such as a Bear, Pearson, Wing, Damon Howatt, Groves, etc. that was 45# the weight was determined by what the bows draw weight was when drawn and measured from the throat of the grip 28" to the string. That was standard for all production bows I thought, and was determined by the Archery Manufacturers Organization, AMO, so they would all have a standard way of determining bow weight since all risers were not designed the same and had different thicknesses through the grip area.
The measurement taken from the back of the riser to the string was referred to as "true draw".
Louis