I believe these weren't standard issue until the AK-74. The stock tang on the AK-74 has the lower rivets about 1/4" farther forward than the AK-47. If you use a 74 rail on a 47, the clearance notches for the pivot pins will be off and may prevent removal/installation of the pins once the rail is installed. You can grind clearance for the pins and everything will be fine.
The other option would be to use a rail designed for the AK-47 rivet spacing. These fit properly and there are no clearance issues. You can pick up rails from Tantal:
tantal.kalashnikov.guns.ru/bstmounts.htmlor Ohio Rapid Fire
www.ohiorapidfire.comSince you're starting with a kit, you can use a rail that attaches to the stock tang with the rivets. On a complete rifle, you would have to remove one rivet or use a rail that screws on.
There are a few other sources online, but you'll have to do some searching.