Transmission identification help

I have just purchased a new project with a Chevy 350 and an unknown transmission. I haven't had a whole lot of time with this ride because a buddy is storing it at his shop until my current project is out of the garage. Its a manual transmission and according to the shift knob it is a 4 speed with overdrive. Where should I look for a casting number? Haven't located one as of yet.. Here is a photo. Any help appreciated!

 

88 and up GM truck?.its not a T-5 since it has a non removable bellhousing.

 

Nv3500.

 

Thanks guys! Looking at some other pictures of NV3500's it sure looks like that is what it is. I'll get a casting number on it to be sure.