Passage of time.

The crank would about 12" down from the top of the machine, usually on the right side. I didn't see it when I looked again at that link.

It could be too dark to see it in the shot or if the phone was a studio prop, it might not even been attached (they are removable by cranking counter-clockwise).

The last possibility is that this machine, even though it's an acoustic phono, could have had an electric motor. I saw a very nice Victrola that had a factory installed AC turntable motor instead of a spring wound motor but were very rare do to the extra cost, and most folks back then were thrifty and didn't mind cranking a phono for 30 seconds to get three record's worth of play time, plus they were trouble free. Many early Model Ts that had a starter motor as an option were also bought without the starter, but relied on the hand crank instead.

According to the following link, the first Victrola with an AC motor was introduced in the teens, something I didn't know:

http://www.victor-victrola.com/History%20of%20the%20Victor%20Phonograph.htm


"In 1913, the first electric motor option became available on the Victrola XVI, eliminating the need for cranking after every few records were played. Victrolas with electric motors were called "Electrolas". This option didn't really catch-on until well into the 1920's, as electrical power was not yet readily available, and the added cost of the motor was prohibitive for most buyers."

So Mary's acoustic phono could have featured a plug in AC motor.

It being black and white though helps with the illusion. Also their oldest son looks like he could be 13 in the climax of the movie. To me anyway. So him looking that old helps with the passage of time imo. At least they aged Potter pretty well. Towards the end of the movie he is so old, he is being pushed around in a wheelchair.