I second that, I am looking forward to seeing side by side comparisons.
I checked the following links for comparison of the letter opener and the 1/1 replica. I think lengthwise the letter opener may be compatible (seems like it is about 1cm shorter than the 1/6 of the replica length) but the grip seems to be thicker when compared with the replica.
Game of Thrones Ice, Sowrd of Eddard Stark | House Stark | HBO Shop
Game of Thrones Ice Letter Opener | House Stark | HBO Shop
I checked the following links for comparison of the letter opener and the 1/1 replica. I think lengthwise the letter opener may be compatible (seems like it is about 1cm shorter than the 1/6 of the replica length) but the grip seems to be thicker when compared with the replica.
Game of Thrones Ice, Sowrd of Eddard Stark | House Stark | HBO Shop
Game of Thrones Ice Letter Opener | House Stark | HBO Shop