If you build one, let me know. Looking at the drawings, it is more complex than it needs to be. Machining the curved breech slot must have been tricky, however I guess the basic contours could have been in the casting since it was bronze.
The gun is in reality a slidiing-wedge breech like Krupp perfected, only the Goodwin wedge had the unnecessary complication of being curved. I'm guessing if a machinist wanted to make something like this, he'd copy a 2-pounder Hotchkiss, or one of the 1890's Krupp guns, any of which used a horizontal sliding wedge with a slow-screw lever to move it forcefully the last inch or so of closure.