Here's a pic I robbed of John Smith's site. It looks like a dipstick location under the oil filter. Is that what yours looks like? The one in John's pic has no freeze plugs on the left side of the engine. It's a very early 9N engine. Later engines had one plug on the left side. I don't know when they started putting more than one plug in the engine.
If it's a replacement engine, that might explain why you can't see a s/n, but that extra dipstick hole makes me think you have an early (original?) engine.
You might want to read through the N-series history on Smith's site and compare what you read there with what you see on your tractor.