I have never heard of the church keeping copies of Mission Calls - if that is what you are implying . . . (Or rather no indication they have made such letters public if they do have them)
There is a LDS missionary database though
I dont see any record there of copies of mission calls.
There is a database for 1830-1930:
I am presuming that his call might have been after that date. I do not know how you would find out for sure. I wonder if you posted a query to the Church History Department
https://chl.libanswers.com/form?queue_id=3148 if they could answer this question.
Please let us know if they are able to provide an answer.