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This is an interesting topic.
I suppose that typically wooden OWT or Granny’s Tooth routers tend to support their irons close to the vertical and have a short stubby base for working across the board.
However, R A Salaman does show a selection of different routers in the Dictionary of Woodworking Tools.
One is converted from what appears to be a short coffin plane (fig 526 a), whilst there is an illustration of a plane (fig 526 e) just like yours, which he called a ‘home made example.’ As it has a long base, I'm tempted to think that it may have worked along the board, rather than across it; trenching a recess with the grain instead of across it.
Possibly it was cut down from another plane long ago. Who know? Jobbing joiners would have cut countless housings or trenches in the 18 / 19th centuries.
Some while back I had a large number of mortise and tenons to cut (by hand) and I wanted to ensure that the tenon cheeks were all the same depth and in the same horizontal plane as the face side.
I thought of trimming along the grain, using the face side as a bed, instead of across, using a shoulder pane, or paring chisel.
I experimented with conventional metal (Stanley / Record) routers but these don’t sit at all well on a long surface – they like to work across the grain and you can't turn the blade 90 degrees. The old Preston routers will do this very well indeed and can have their blades turned at right angles, but I had three different depths to cut and it would have meant continually altering the setting.
I solved the problem by utilising some redundant plough irons and making my own long router plane. It's a bit of a hybrid, but was very successful, so I made another. That meant that, in combination with the Preston, I could have three different depth settings throughout the job that remained the same.
I wrote about it with more detail in the blog here, if you are interested. The pictures explain the process better than I can here:
http://handmadeinwood.wordpress.com/201 ... an-outing/
I should add that I made a later post in the blog about how the long router was made:
http://handmadeinwood.wordpress.com/201 ... on-router/
It’s probable that the old joiners had two or three (possibly homemade or converted) router?
I found that it made a big difference to the way that I could turn out well fitting tenons consistently and quickly, and it's not far removed in concept from the original that started this thread.
All best from Wales