The serial number decodes to when the frame was built.
The number on the head badge decodes to the day the bicycle was assembled.
1972 frame?, (69/79) head badge?
Schwinn did keep some frames on hand for warranty replacement purposes but for 7 years? Since your tandem was repainted, I'd say the badge is from a different Schwinn (maybe a Varsity).
odd Serial number. SH5866, hand stamped on the bb shell during the time when Schwinn put their serial numbers on the head tube. 1952 was the last year for bb serial numbers but they weren't making the Twinn then. (and the Town & Country is very different). The 1952 era serial numbers only had 1 letter.
Using the 1965+ system, A=1965, B=1966, etc, H is the 8 letter of the alphabet so that would be 1972. But the first letter S doesn't work. A=January, B=Feb, C=March...
(they skipped I) S=18th month of the year? And only 4 digits where there should be 6. Is this an added serial number? Is there another serial number on the head tube at the bottom? It might be obscured by the repaint. Perhaps they found a really old stock bb shell when they built this bike. Certain models did get serial numbers independent of the bulk of the Schwinns. The fillet brazed bikes and Paramounts, the cycle truck. So did Schwinn use a different serial number system for the Twinns? There are plenty around yet so perhaps some owners will let us know where & what they have on thiers.
Sprockets are all not original. The rears look like they are off an old varsity, the fron from an AMF bike.
Rear pedals not stock.
Newer saddles.
Tires are replacements. Wheels look to be off some other bike.
bottom line, a franken bike. Not worth much and surely not worth restoring to showroom. It's really a blank canvas for you to do anything. go nuts with it.