The following references describe mathematical printing in the 1950s, before it became too expensive and degenerated into illegible typescript:
The Printing of Mathematics by T. W. Chaundy, P. R. Barrett and Charles Batey, Oxford University Press, 1954.
Setting Mathematics by Arthur Phillips, in The Monotype Recorder 40 (4), Winter 1956.
Printing Mathematics by Peter Basnett, in Eureka 23 (1960) 11-13.
At that time, ordinary text (novels and newspapers) was set using the Linotype system, in which the compositor would use a huge keyboard to select characters and then they would be set in molten lead a whole line at a time.
However, this was unsuitable for mathematics and the "state of the art" was the Monotype Four Line System, which had movable type. The following piece of nonsense is a block provided by Monotype and used in the Eureka article:
Each letter is set on a block (shown in grey) that occupies space on one of the four lines. However, the $g$, $X$, $b$ and $Y$ overhang their blocks and are supported by spacer blocks (white) in the other line.
You could add big integral or summation signs in front of such a fractional expression, and (I think) a $d x$ vertically centered after it.
Here is the Eureka article but I will happily delete it if the author or current editor objects.