I need to replace a lock for my back door

The only way we can make a recommendation is to use the original handle set as a reference. However, much of yours is missing, and I don't know what your dimensions are at this stage. 

Based solely on what I see in this photo, my top suggestion would be the 40-049, but this is a guess. Your next step should be to carefully examine the dimensions we show for this product to determine if they match up.

alternate view of 40-049

Dimensions for 40-049

This photo does help; it tells me that you have the same style mortise lock as what the 40-049 uses. While I'm still not 100% sure that it will work, I am more encouraged in my recommendation. I definitely think it's worth considering.

I'm afraid I don't have a phone line; we don't offer a phone identification system. I can only help you right here on this page. That said, there's not much I can do for you at this stage. Your next step needs to be to carefully review our specs for the 40-049 and determine if you think it's worth giving a shot. Just understand that it's not a guarantee. You have a very old security door and odds are that the exact handle you need no longer exists. I figure that you're going to take a gamble no matter what you use to repair this door.