Charlie,
I come from the electrical end of the engineering spectrum, but I worked 37 years for a metals manufacturer and after thinking about it for a few minutes I thought I might have seen that symbol on mechanical drawings. First, how I made it:
In Pages, I created two Text Boxes. One containing a large letter P and the other containing a large letter L. I set both text boxes to Floating and turned off the Object Causes Wrap control for both boxes. Then I positioned them and grouped them (Arrange > Group). Next Command-C to put the combination symbol on the Clipboard and switch the focus to the Preview.app program. In Preview, Command-N to make a new file from the Clipboard Contents and save it in whatever format you need. To post in the forum, I used PNG, but for use in a table, I'd use the PDF format. If this symbol is going to be used in a sentence, it's more complicated since you can't easily mix graphics and plain text in a cell with good position control. Better to compose in Pages using the symbol as an Inline Object, convert to a PDF using the procedure above, and then paste the PDF into the cell.
Regards,
Jerry