Yes, it appears they had a relationship. I hate that historians have minimized that. I love the chemistry too. I agree, in a room filled with people, they seem to be only ones and really know each other. They see the other person.
Check out this book. It not typed out like a typical book. It’s “how she intended them to be read.”
Emily Dickinson’s Poems: As She Preserved Them https://www.amazon.com/dp/0674737962/ref=cm_sw_r_cp_api_glt_fabc_NR9E5XWPJVD5K7SN4G49
You will have to forgive me, I have done some searching but do not know the answer to your question. It has been some years since I have been active with Dickinson and I am the only active moderator here.
With that said, I do have some suggestions you might find interesting. First of all, there was a book published that could make a wonderful gift, entitled Open Me Carefully which no doubt includes the letter. In addition, if you happen to want to browse through the letters Online, there is the Dickinson Electronic Archives, which include Correspondence with Susan Dickinson, including images of the letters, and most of them with transcriptions next to the images. If you can find the letter itself, you could print out a portion of it and perhaps frame it, or use it as a gift some how?
I wish you the best, and please report back with the letter if you are able to come across it. Thank you.