No offense, Mike Irwin, but you're a bit wrong there. The standard military pistol before then was the Nagant revolver. The Tokarev was the first semi-automatic pistol accepted into Soviet military service. The military Tokarev loading was indeed hotter than .30 Mauser. Some reloading manuals used the same loading data for both simply because the loading information of .30 Mauser was well-known and was known to function fine in a Tokarev, whereas information on the Soviet loading was not well-known. This does NOT, however, mean that they were the same in practice. The Tokarev round was indeed hotter. Note that this round does not gain much velocity out of a longer barrel; its powder charge is very much optimized for the shorter barrel of a pistol, and it winds up coming out of an SMG or rifle barrel with about the same velocity.
What DID happen was the Germans ordered a ton of obsolete .30 Mauser ammo to the front lines for use in captured Tokarevs. Russia may have intended that they be able to use captured ammo, but the reverse wound up being true because they captured so many Russian pistols but not much ammo.
I do not believe the .30 Tokarev / .30 Mauser backwards compatibility was intended, however. Keep in mind that, back then, Russia had not really developed an "Us versus the West" mentality. This did not really occur until after World War II. However, 9x18 was most certainly intended, as is the recent 9mm Luger adaptation.