Coltrane,
Again, Congratulations on your "rescueing" a Finnish MN39! The 'vkt' mark stands for Valtion Kivaaritedhas: The Finnish State Rifle Factory at Jyvaskyla, Finland.
Virtually all Finnish Mosin-Nagants will have a captial "D" stamped on their chambers (generally the buttstock as well) to indicate that they have been chambered for the Finnish D166 cartridge, which was developed to take advantage of the huge amount of captured Soviet "D" ammunition. (Terence W. Lapin)
The Czech ammo is usually very good and later manufactured rounds have not proven to be corrosive, BUT I RELIGIOUSLY CLEAN THE 91/30 AND MN44 AFTER EVERY SHOOTING. NO REASON TO TAKE A CHANCE AFTER ALL OF THE HARD WORK TO GET IT BACK TO BEING A SHOOTER.
If you have other proofmarks and stamps, possibly I can help you in deciphering them. Be sure to indicate whether the mark is on the receiver or barrel OR on the stock.
If you have a hexagon receiver on your MN39 - that alone instantly increased the value of your rifle. The Finns did not manufacture a Mosin-Nagant receiver as they had huge stockpiles of Russian receivers on hand. To all intents and purposes only "Type 1 Hexagonal" receivers were used on Finnish rifles, as these were the only ones in stock up to the Winter War of 1939-40. Even after Russia attacked Finland on November 30th, 1939 - there remained sufficient quantities of the hexagonal receivers in the Finnish inventory to preclude the need to manufacture any. (T.W. Lapin)
Having both SAKO and VKT on your rifle (they are two different Finnish rifle factories) might indicate that the rifle was assembled from parts of other rifles. This could prove to be a very interesting little mystery on your purchase......
Take care and looking to hear more about your MN39.