First off; I can imagine how annoying questions like I'm about to ask my be...
I recently purchased a single-speed bike at a LBS. It was a trade-in. The previous owner had done a rather decent job of converting the frame to a SS. However, they had stripped the finish with a chemical bath for the "patina"(?) look. The head tube doesn't appear to have had any rivet or screw holes for a head badge. All I have left is the serial number on the BB. Initially, I wasn't that concerned about date and company of manufacture. But, the more I've gotten to "know" this bike, the more inquisitive I've become. I've searched all my known avenues of info to no avail. I've posted pics of the bike when purchased and after my paint job and some changes. Any ideas what brand this frame may be?

...more photos of unknown bike. Thank you so much for any info or direction ideas. BTW, the frame did have remnants of braze-ons on the down-tube and the right rear chain-stay; gearing I'm guessing...

Nice photos, but you haven't really given the folks here much to go on. Unfortunately, there are myriad schemes/formats for serial numbers. Unless someone recognizes the pattern as a distinctive one, that won't be too much help without other salient clues. Good photos of the area around the seat post entry, fork crown and front dropouts might help a bit. The rear dropout does not appear to be forged, which suggests either that the bike is fairly old (1960s), or a cheaper model. I don't recognize that one, as a mass-manufactured item, but that doesn't mean much -- maybe someone will.

It's tough to tell from the photos - are those wrap around stays at the seat cluster? I agree that the rear dropout isn't forged, but the connections look to be better constructed than typical gas pipe frames. I'm speculating older British, based upon the limited visual information. 68mm English thread bottom bracket? Let's see a detail of the seat cluster and the BB. What is the rear spacing?

Kuwahara marks their bikes with a K. Probably robot welded Ishiwata tubing.

more photos as requested... Thanks for your help

Some ideas. Has a kickstand plate, so likely not a higher end bike - more of a utility type bike? You could try tapping the top tube with something along its length and listen to tone to try and ascertain if the tubing is butted (thicker close to the lugs and thinner in the center).
Measure the exterior seat tube diameter, then also measure the seatpost. Tell us the numbers. My understanding is that it will tell you how thick the tubing is, which can give more information.

We're the bottom brackets standard threading? Also, sometimes the steer tube has markings on it. Did you look for markings when you had the fork out?

I looked into this but my serial number starts with a GK, as in GK9G11432... Maybe a Giant or GT?

The fork crown, seat stay terminations at top, seat/top tube lug and bottom bracket lug are not "crude" but don't look like those of a higher-end bike. Another clue to the quality/buttedness/lightness of the tubeset is the (proper) diameter of seat post that the frame takes. Yours looks appropriately sized, based on the slot at the rear. None of these factors will indicate a manufacturer, unless they're very distinctive (not the case for this frame, I think); but they do indicate frame/build quality.

Don't be disappointed; I commute on a frame that has no obvious "cachet", but it's very well constructed, and I enjoy it for that. Other frames have a better ride and pedigree, and are one or even two pounds lighter, but this one is special too. It has a "stately" presence. People love their old Raleigh 3-speed models and Lentons, even though they were not the cream of bicycle offerings in their day.