I always too though of it that way. In the center portion of the information they provide where the specific gun was shipped. Then at the end they state, "Number of same type guns in shipment." To me that would indicate the number of guns like the one you had lettered were sent to that same point of shipment.
If I may jump in,I have a .45 Bisley shipped to the Copper Queen. The letter indicates it was a 38-40 originally.It was a shipment of 5.It does not appear to have ever been altered and i believe the letter to be in error.(not an uncommon occurance)I have since found a 45 Bisley shipped the same day (shipment of 5) to the same location which helps validate my theory,but will forever leave a doubt.
Your are correct on the total number at the letters end
Here are 3 factory letters showing more than one of the same type guns in shipment.
That's how many were purchased by & sent to the colt dealer / military