The stock finger groove changed to the long-cut sometime in the letter series of 1959/60. Seeing that we dont know the order in which the letters were produced, its going to be hard to determine the letter in which the transition was made. More then likely we may see a random transition with two or more letters having both variations. Definitly something to keep an eye out for.
This cut so seems to coincid with the wide font transition. The current observed letters containing the wide-font are B, C, D, E, F, H, R and W.
Notice how one of Marshalls rifle is an F and the other is an M...... willing to bet the M has the narrow groove stock.