w44wcf,
I wouldn't know without checking the serial number but my best guess is that this gun has been cut. If this '73 were special ordered with the 2/3rds magazine and 21" barrel, it would definately be listed as such in the warehouse ledgers. Sometimes when a bunch of guns with the same features are listed on the same ledger page, a quotation mark or a check mark will follow the first gun right down through that column for all those consecutively numbered guns. In other words, the first gun on the list may have the barrel length written down but others that follow may not if they are in fact the same length as the first one. In many cases, depending on serial range, if the barrel length is not listed, this means it was the standard length of 24 inch.
Adios,
Buck