Huffy
OK, I took some time on this and my answer still stands at Mid 1960's (1964ish) middleweight built by Huffy. Seat & grips not correct, probably originally had a rear rack. Here's a comparison pic why I still say Huffy & this took some time to do so here it is. Keep in mind I am NOT an expert on these middleweights & hopefully a real expert will weigh in. You might want to post the SN.
Also that AMC Flash (not AMF Flash) rideahiggins posted looks Huffy built to me. I have a 1961 Huffy badged Eldorado with the same tank, same exact graphics on fork, & same chainring. I have NEVER seen that chainring or chain guard on an AMF bike. I even went back & looked through every AMF bike pic from this era I have saved & on google, nada/none have it. Both are on various Huffy built bike pics.
The frame is what has really thrown me off on this bike & looks like a taller frame than I've normally seen on mid 1960's Huffys but I think that is exactly it, it is a taller frame version. It even has an extended seat post.
On the Hiawatha name, Hiawatha bikes were sold by Gambles Department Store exclusively. They did not build bikes but bought them from various manufacturers such as Huffy, Murray, Shelby, etc. throughout the years.
OK experts, go ahead & rip this apart as I'm looking to learn too, but to me this appears to be a Huffy manufactured bike.