Take care! there are different versions and sizes of 7x5 darkslides.
Firstly there is the obvious difference between half plate and 7x5, but also I have six darkslides which take 7x5 film exactly, and six which are 2 to 3mm wider, which causes the film to fall out at one side. Both types look identical on the outside and are marked Fidelity.
I discussed this with Sue Evans in the tech department at Ilford / Harman and sent her one of each to prove my point. Her conclusion is that there must be some cheap and inaccurate imports coming in from somewhere.