"Forward-" and "Backward-" Angled Eccentric Cranks

Most steam engines have the valve gear eccentric crank angled toward the front when the drive rods are in the bottom quarter. However, some engines had their Walschaert valve gear eccentric crank angled toward the rear, resulting in the valve rod (connecting to the combination lever and valve stem) being in the upper position on the link for forward motion. Notable examples were the Canadian National and Grand Trunk Western class U-1 4-8-2s (except for the bullet-nosed U-1-f class). See the photo my brother took of U-1-a 6014, here: http://www.railarchive.net/cnrgtwdvl/cnr6014_dvl.htm . My question: What was the reason for this fairly rare arrangement? It probably was never used for new locomotives after 1930.

Think it was mainly Alco that liked the indirect Walschaerts-- they said the eccentric rod and main rod angularities tended to cancel out that way. (Speaking of inside-admission engines, that is. I guess lots of outside-admission engines had leading eccentric cranks?)

Most of these CNR-GTW Mountains came from Canadian Locomotive Company or Baldwin. Only the U-1-e class (1930) came from Montreal Locomotive Works, Alco's Canadian affiliate.

Of course other U.S. railroads also had engines with this "backward" eccentric crank. If I run across any examples I'll cite them.