The font most often used is Gill Sans - which was designed in the 1920s, but it seems to me that the original poster is more likely to have been hand lettered, or printed using letter press lettering. This is how it was done back then - poster designers could render lettering to a very high degree of accuracy.
From Wikipedia - a scan of the original poster:

In fact, if you look at features such as the upper case R they are close, but not spot on. Also see how the letter C differs. Here is the word “carry” set in Gill Sans:

And in case you thought that another popular and well known font would substitute well, here is the same word set in Johnston (the font used for the London Underground)

So, yes, even Gill Sans is wrong, yet it’s become the defacto font for these kinds of posters.