Math; 48 is 1/2 of a ratio calculation 48:24 is 2:1, for example..




You have a double chainset that someone left the inner chainring off of to have a single..

the basic model was a swaged on crank arm, you had to pony up more money, for cranks with 2 replaceable chainrings
and BB with a Bolt in a hollow spindle rather than a nut on a solid steel one...



By the way, to my eye, it's too old, & lower end , you have a Screw on Freewheel , not a Cassette freehub.


....