According to Bert’s excellent article on the model 71 in the Fall 2013 Winchestor Collector magazine (page 22), it states “Receiver frames that were factory drilled & tapped for peep sights.  This change took place in late 1947 to early 1948, in the 24300 – 26000 serial number range.  Receiver frames were available with the drilled & tapped peep holes on special order from the beginning of production, but they must be carefully examined to determine if the holes were factory work.”

Don