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.