Hi,
There are a number of weston saws that are back saws, they all have a 'back' fitted to the blade, this adds stability to the blade helping to keep it straight in use. On the bigger back saws the extra weight makes them cut faster without needing the user to try and push down on the blade.
The back can be brass or steel, some are folded over to form the slot for the blade while some have the fine slot machined into a solid block.