It's a bread slicing guide with two thickness options. Just print the guide as one piece and print the stopper separately.
The stopper just drops into either end, depending on which thickness you want.

I wanted to be able to cut different sizes for sandwiches or toast - so the two thickness are 'medium' (11mm) and 'extra thick' (15mm). With fresh bread, I found it difficult to cut any thinner.

To try and make it robust, it is quite chunky (250g+) and there is a fairly large 'knife gap' to make it easy to use without damaging it.

The short version is just so that it fits into our bread-bin neatly, and is fine for small / medium loaves (from a Rusell Hobbs compact breadmaker). The long version should be better for large loaves.

Don't print the base_mask file, this is just to enable strengthening (see print settings).

There are different opinions on how food-safe different materials are - please look into this before deciding to use it.