>This is meant to be an egg-sized quantity of butter, but what was a normal sized egg in 1905?

This site [1] has some interesting info:

[1886] "The average weight of twenty eggs laid by fowls of different breeds is two and one-eighth pounds. The breeds that lay the largest eggs, average seven to a pound, are Black Spanish, Houdans, La Fleches, and Creve Coeures. Eggs of medium size and weight, averaging eight or nine to a pound, are laid by Leghorns, Cochins, Brahmins, Polands, Dorkings, Games, Sultans. Hamburgs lay about ten eggs to a pound. Thus there is a difference of three eggs in one pound weight. Hence it is claimed that in justice to the consumer eggs should be sold by weight." ---The Grocers' Hand-Book and Directory, Artemas Ward [Philadelphia Grocer Publishing:Philadelphia] 1886 (p. 67)

With similar figures given for 1911 as well. Which would suggest a normal egg in 1905 would be approximately 56g (1 pound/ 8 eggs = 0.125lb per egg).

[1] https://www.foodtimeline.org/foodeggs.html