I think bond-o is right as the numbers are 1 50 B, at least that's what it looks like to me.
The serial number looks like OB415926.
The drive should be vintage 1986.
Yep, it's an Alpha I. Here is a link to the free online manual if you don't have one. With a SN, you can be sure that some previous owner didn't change it to an older drive. It's been done, trust me.
Look at page 3 at this link.