A little licence exercised by EB. While British and Commonwealth graves had a bow-topped stone (in many cases incorporating a religious symbol in the inscription) the French used white crosses extensively.

The hypothetical WMcB would more likely be found under a British-type stone, but there are certainly plenty of white crosses to be found.

A few images.

"White crosses" makes the point quickly, effectively, and brings to my mind some powerful photos. The slight technical quibble is just that, and does not detract from the song in the least, IMO.

My main regret about the changes that have occurred over time (apart from the butchery committed by certain performances) is that EB dropped his earlier reference to "the bugles sing the last post in chorus"; a fine choice of word, and most apt.