There is a "39" stamped on the bottom of the barrels .... but there was a laminated tag attached to the trigger-guard from the former owner that said ..

"caliber 500 3" express"

and even had loading data with IMR3031 and IMR4198 with velocities - HERE ARE PICTURES OF THE TAG

https://www.buckstix.com/buckpics/GRANT-HANG-TAG.jpg