Actually I have absolutely no inclination to set up & pour chamber casts of a slew of old doubles just to find the exact length of their cones. With a "Practised Eye" though I can look at the shadowed cone & state positively that none of mine have an abrupt step nor do any of them have a "Modern Long Cone".
My observation thus is that 5° cone angle (10° included) is just as applicable to a 1915 (or earlier) gun as a 2015 one.
There is simply nothing "Modern" about that cone which is all I have been saying this whole time.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra