Inst Code

07:59 Mar 13, 2020 

It could also be "instance code" , used in certain databases but here it gives refers back to the code given in the database and is generated from the cheque number.
I agree it's probably already in English so if we can't find an answer, I would simply translate it as "Inst. Code" (with a dot for the abbreviated word).

02:55 Mar 13, 2020 

I'm pretty sure "inst" = "instrument" here (a check being a financial instrument). In the link I posted in the first discussion, the document is virtually identical to the one I'm translating, just "Numéro de l'instrument" appears where I have "Inst Code".
Even so, Sanjin is right - in French it would still be "Code de l'instrument", but I too have seen stranger things before now. Besides, oddly, many of the abbreviations used in the descriptions of the transactions I'm translating are in English.

02:40 Mar 13, 2020 

"Any ideas? "inst" = institution?"

It might be an "institution" but in French, the word order should be " Code Institution ou Code de l'institution", therefore it would make more sense if you had to deal with "Code Inst" instead of "Inst Code". Just a hunch, I work in a financial services industry and have seen stranger abbreviations, so you might be right as well -:)
Good luck with this one.

00:07 Mar 13, 2020 

The inst code is the same as the cheque numbers to the left. But I still can't think what it might stand for

19:39 Mar 12, 2020 

I think I may have found this: "instrument"
https://www.scribd.com/doc/230319219/187822-1400678767406