Hi Tom, Neat find!
What you have is a Page model ZA12 Master.
Page's first rear engine, riding tractor was based on the Walk Behind tractors that they sold for years prior, the first model manufactured beginning a model ZA10 in 1946 used the Walk behind tractor drive train as the rear end, that they just simply added a forward/reverse selector to. It still utilized the walk behind tractor's individual wheel clutches which surely makes it interesting to get used to driving, but it will steer really short being able to utilize the individual clutches with the steering wheel. That first model was considered the "standard" model and was powered by a number of different 4-5hp engines, but most commonly, the Wisconsin ABN. This model didn't have any brakes.
The transmission is unique and specific to these riders, however the rest of the drive train is the same as a walk behind tractor which you can find easier to use for parts if necessary.
In 1950, Page introduced the "Master", which was a slightly beefed up standard model. The front casting was redesigned to be heavier, it had individual wheel brakes and the biggest improvement in my opinion was a hand operated clutch and differential. It was powered by a slightly larger Wisconsin AKN engine. Page produced both standard models at the same time for a couple years and the Master was produced through 1957 until replaced by Page's new lineup of front engine riding tractors.
Your tractor has been re-powered by the AENLD. The only thing I've seen that's been consistent on the Master models, is the use of the AKN engine. To confirm, the AENLD on your tractor dates to 1967. Based on the pictures of your tractor and it's configuration, I'd estimate yours to have been manufactured somewhere between 1951-1956. I also see that your brake pedals are missing which could be remade to be functional and look close. It looks like someone added some stuff, but can't make out what was added by the pictures. The biggest thing missing is the equipment lift. Otherwise, it looks like your tractor has "good bones". I like the steering wheel yours has which I believe to be original.
They are scarce, both the tractors themselves as well as parts and any related information. I've managed to collect (4) Page rear engine riders so far; 2 Standard's and 2 Masters.
I'm happy to help you out anyway I can.
Thanks, Rob