Looks like a tiny bit of shorthand? There's a lot of shorthand in Roosevelt's papers.
I would like to offer a possible bit of insight from out TR curator. I passed your question on to her, while she is not completely certain, she said:
The only other thing I can think of is that it was a little flourish by Loeb, like his “brand.” Or perhaps a symbol he made up for his own use?
Noting the symbol [?] is completely fine. Thank you for your attention and effort!
All the best,
I am not quite sure if the mark, in the first picture, on the left, is a symbol on the chart at the above link listing printers and proofreaders notations, symbols, and marks.
Do you have a link to the page you are referring to?