The # sign can mean a few things
# ie lbs in this case lbs of force before it breaks,
I use it at work for "square unit"......unit typically being a foot of measure, meter, yards etc could also be used as the unit of measure.
My first option, ie lbs of breaking strength, could in some instances, be kilograms or equal force measure too, but lbs is most common.
She drives me boat,
I drives me dinghy!