Ive not seen one marked with 2/3 mag or 3/4 mag on a carbine forearm.  The reason for the 1/2 mag designation on carbine forearms is because the angle of the magazine tube bore is steeper, allowing the end of the magazine tube and cap to engage the barrel.  You may be able to get by with a standard carbine forearm for 2/3 or 3/4 mags as the magazine runs parallel to the barrel over a longer distance.  Just a guess.

Havent ever seen a rifle forearm marked with the 1/2 mag, and probably wouldnt expect to, the wood appears the same, the forearm cap is different, the angle of the magazine tube bore would be the same for standard and button mag rifles.  For 2/3 or 3/4 mag rifles, I suspect it could be a bit different bore angle, not sure.  Below are some pics of forearm caps they are milled to accommodate the various magazine lengths or angles.

L-R:  RBFM, OBFM, OB2/3M, OBBM, RBBM

Kind of like old  Bert, I dont make it a habit looking for stuff, but curiosity oftentimes outweighs the risk.  Youd be surprised what you sometimes find. 

LOL!!! 

The way I look at it, if your not tinkering your not learning.  No guts, no glory. 

Chris