With the body rabbeted out to accept the side plate, it’s time to cut the actual mouth.
If you haven’t yet, now would be a good time to cut the metal pieces to final length. Since the O1 hasn’t been hardened, it cuts pretty easily. I did the first two planes with just a hacksaw. For this one, I used a hacksaw on the 3/8 and a jigsaw on the 1/8.
Lay the piece of 1/8 steel on the bench and the wood blank on top and tightly nestled in the rabbet. I hope your wood blank is still a bit overlong. If so, let the wood overhang the steel just a bit on the front. Add the 3/8 steel standing up tight against the wood blank and use a sharpie or similar to mark out the mouth opening.
Pull the steel and you should have marks ending 3/8 of an inch from one edge of the plate and extending to nearly the other edge. With a combo square or other 45 degree tool, mark the opening on this edge as well.
Using your metal cutting tool, cut out the mouth. I used the jigsaw again and am much happier with the results than the first two planes that were done with a hacksaw. It worked fine, but the cuts were very rough and required a lot of filing to clean them up. The cleaner the cut, the less cleanup will be needed later, but always better to leave some of the line and sneak up on a good fit later than to over cut and have to scramble.
Set the wood body back on the steel and check your fit.
Now is also a good time to double check the body thickness and make any adjustments.
Once you are satisfied with the fit, the frog can be attached. If you still have the screws from disassembling the transitional plane, those should work great. If not, round head wood screws can be purchased and used.
Next installment: Tap, tap, tap away.
-- Pay heed all who enter: Beware of "the Phog" Rock Chalk, Jayhawk