The bridge itself is 5/16″ thick, so I lay out a second line 1/4″ above the bridge line – this line is 1/16″ shy of the top of the bridge to allow for a shoulder at the top of each tenon – then I place the mortises within the space I’ve defined. I’m planning to use a bridge that is about 1-1/2″ long, so I lay out my mortises 5/16″ wide with 1/2″ space between them.
Now, I cut this layout out for use as my template, paying special attention to the index lines, which are the front and top edges of the sidewalls. These are what will ensure I’m laying the mortises exactly symmetrically. First on the ‘left’ portion of the sidewall :
If you look closely, you’ll see that each dovetail is actually only ‘flared’ in the lower 1/8″ or so – the top 3/16″ of each joint is perpendicular to the sole. This is the first time I’ve seen this on a plane – my best guess is that Towell used this technique because it greatly reduces the size of the gaps at the top of each joint, which must be peined shut later on. With such a large sole, a full dovetail would be quite a chore to close up, and is really not necessary for strength purposes. I think this is somewhat of an aesthetic compromise, but it is a very interesting feature and I’m going to emulate it here; since the plane is steel-on-steel, the dovetails are not particularly visible after lapping anyway, and it strikes me as an interesting technique to try out.