End Doors

The end doors in boxcars also provided ventilation. I agree with CJ the lumber answer is correct for both species of cars as the most common use of the end doors. The stock cars were used seasonally for stock movement, but the railroad would have tried to use them for other revenue use other times. I'm sure the smell of a ripe stock car was not popular among customers paying for hauling fresh cut lumber. Stock cars were even used for hauling coal at times.
I would have guessed 1940-41 as the years of real transition to the flying heralds. May well have been adopted for use as early as '39, but I don't think it was common on the narrow gauge until at least 1940. By 1941, it's everywhere.
There was a discussion some time back on the origin of the Flying Herald, and I was surprised how early it was. Look at pictures of the Chili line and the Shavano for clues. I seem to recall both disappeared in '41 and the flying heralds were in abundant use by then.