Checking back in with C&V after a long hiatus ... I feel a little guilty, but I know this is a forgiving group!

A co-worker biked to work on an old beater. He explained it's the spare "house bike" that he and his roommates use as an extra.

On closer look, it looks like a high quality frame.

It has no headbadge, decals or stickers - no identifying markings I could find.

Except for this AD logo on the seat stays.

I'm sure someone here knows: what kind of frame is this?