I can't seem to find any information more specific than this, and considering that all of the sources I've found seem to be parroted from each other, I'm wondering where this little factoid originally came from. What trades used the "flock" as a measure? What was it used as a measure of?flock: an old English unit of quantity equal to 2 score or 40.
The best lead I have (and indeed, the most intuitive one) is from the likely origin of the word "score," which was related to "counting large numbers (of sheep, etc.) with a notch in a stick for each 20." This suggests that two score sheep might have composed a standard "flock," which makes sense, but so far I haven't found any primary evidence that the word was actually used in this way.
If anybody knows of a reference that could help me here, or could point me in the right direction, I would greatly appreciate it.