I'm the OP on this and here is what I just took down in my barn because I'm having it insulated. It's iron pipe, I put it in 20 years ago and have no idea where I got the plans. It's worked fine for me.
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The hose off the connector went into the long pipe on the right that was also a drain. The down pipe on the left is also a drain pipe. The center pipe had a filter and moisture remover and my output hose connected there. This worked well for me. I drained my tank and filter every day when I used it and no problems.
My plan was to ditch this and replace with something new and easy to install but now I'm wondering if I should just put this back up with a new filter and moisture remover and call it good.
These days I just use the compressor to pump up tires and blow off equipment. I paint a few pieces of ag equipment a year and use OEM acrylic enamel paint. I only paint on dry summer days when the temps are right. It's low humidity here and I rarely use the compressor in the winter.
After all the comments, do you think I should leave well enough alone?