How can I find this letter from Ambrose? In more than one work I encountered the following format:

(epist. 50[25],2f)

or

(epist. 49[59],3)

In referring to letters(?) written by Ambrosios of Milan, "epist." is explained as "Epistulae." which seems obvious.

But what does this indexing scheme refer to?

I would like to read the letter in the first example in either English, German or Latin.
After hours of additional search I found part of the answer in a footnote of Thomas O Loughlin's "Celtic Theology", p. 50. It seems to be the case that the number in square brackets usually refers to the ordering according to the "seventeenth-century Maurist edition". Other numbers mentioned are typified as pertaining to the "ET" or / and "FC series."