Like db's excellent information my input is only a guess. The model 25 was Lawrence's top selling holster, sort of a flagship for the line. The later Challenger holster was a cheaper similar holster that Lawrence stocked, the model 25 remained a made to order holster available with many extras.
My guess is that your holster is pre WW II. Warshal's holsters were made by A.E. Nelson after the war, the description of the revolver the holster was made for is characteristic of earlier holsters. The post-war pre size code Lawrence description would typically be "M 6 1/2" like on this post-war model 25.
Again, just a guess.
Regards,
turnerriver