Hi, I noticed that no one gave you an exact answer. To measure the existing rack you would need a tool, thread measuring tool, they get expensive. The cost of the ones for bolts aren't bad but rack measuring tools are more common to a machine shop and not the home shop. That said If you are matching a rack to an existing pinion or gear you have to be; exact.
I found that my Seneca wood cameras have the same racks as a camera I built many years ago. It was from Small Parts Inc. Brass Rack 48 pitch 20d PA. My Kodak 2D has a different set all together.
I'm building a 5x7 and have ordered the racks and pinions and have them ready. Two 24 inch racks that are 32 pitch 3/16 inch face, and 20 degree pressure angle (PA). The pinions are called spur gears and have a 32 pitch 16 teeth, .5 pitch dia.
The racks were $56.44 for two. The gears were $34.92 for four.
It's expensive but I will have close to two feet of extension and since I have a new Canham bellows and two fine planks of old Mahogany it's worth it compared to what a Canham 5x7 costs.
Back to your question about what rack to get. You need to take it to a machine shop or you can order some gears that match the gears on the camera then you will know which rack will mesh with it. Give Small Parts a call and talk to their tech. rep. or call Boston Gear. The racks and gears I bought were actually made by Boston Gear. A machinist can tell you right away what it is though and would be a good way to go.