Thanks to the above for the help!
I posted some new pictures; it's been pretty cloudy outside but it cleared up a bit today.
As you can see, there is no maker's mark on the receiver. There's actually not a single Japanesecharacter on the entire rifle. The only thing I could find was the two-digit number "60". Arggh! I wish I had asked the seller where he had gotten it!
Hope that helps!