I've hesitated to comment here until I talked to my friend today who is THE M77 expert. He says the 7x57ST was only made in the 22" lighter barrel, same as the 30/06. Only early M77's to have the 24" medium weight barrel are the 257 Roberts, 25-06, 7mm RM, 300 WM and 338 WM.

El Numero Uno
a Serious Collector of the Ruger No.1 rifle;
a Modern Classic Sporting Arm