Great info everybody, thanks!
To sum up:
Originally (best guess) a 1947 Schwinn DX
Evidence:
• serial number on BB tube only, predates known Schwinn serial number records
• welded-on, un-tapered kickstand- first year is 1947
• frame, fork, stem, handlebars, kickstand, fenders all believed to be original equipment
Bicycle subsequently rebuilt or modified by later era components & repainted
Evidence:
• Non-standard 1-color paint scheme
• Later-era Schwinn components: S-7 wheels, chainguard
• Non-Schwinn components: grips, seat, crank, pedals & sprocket
• Missing components: tank, head badge
Conclusion:
• Could keep & ride as is (needs tires)
• Could keep & customize
• Could part out (possible)
• Could restore (probably not a reasonable option)