This was intentionally done with some frequency (for reasons I don't know) using chemical(s) in the late 1960's and early 70's. Interestingly, I've only seen this done on $1 notes before so I feel you have something of a rarity by comparison.
I had this situation on a 1966A $100 red seal. The back, the red seal and the serial numbers were all bleached out yellow. I bought it on a Heritage Auction. They advertised it as "Better living through chemistry". I had a hard time selling it a few years latter. I lost about $50 on the deal.
Oh my, this is very common by scammers (or wannabe scammers) with red seal $2 notes. I haven't heard of this being done on a $100 red seal before, though! This unethical behavior with the treatment of red seals is abhorrent as the notes are essentially ruined from being permanently altered as a result of being chemically treated.
Green currency ink is made of yellow and blue. One is soluble in acid, the other in alkaline. So a blue or yellow back is yours. Use vinegar or bleach, depending.
I submerged two new $1's .....one in vinegar, one in household bleach, and let them soak for one hour. Neither bill had any color change whatsoever......on either side. ZERO color change. I rinsed off the bill soaked in vinegar and it's dry now and just fine. The bill soaked in bleach disintegraded into a mush consistency and is now in a ball shape. It literally fell apart in my fingers and I was unable to lay it flat.
Every note I've ever seen that's been altered to have an orange back like the one in the OP's post has dated to around 1969. I may have previously seen one reported from series 1974 but I can't recall for sure. Perhaps whatever chemicals were used to do this were phased out from availability in the 1970's? This could explain why later series haven't turned up like this. That or perhaps it was just a fad at the time that was short lived.
In the 50's & 60's there were photo-copying machines that used ammonia. SC & US notes changed color when sent thru this process (if I remember correctly).
