The underwhelming offer of $70 may be related to the fact that the image of your note is very indistinct. You "believe" the grade is Unc or better and give the impression that this note is somehow special by how it was saved from reaching circulation. But every old note that is truly uncirculated is a freak in that regard. Not only does the note have to be uncirculated when it is put aside in the past, but it also has to remain that way throughout the decades. Lots of stored notes get damaged or mishandled and end up in quite imperfect condition when they show up for their inaugural offering to the collector market.
So your note's image makes me think your note could be in any grade from VF to Unc. The contrast is too high and the edges look cropped. Most people don't take good pictures of notes. I would estimate that 80-90% of all banknote images online have the brightness and/or contrast cranked up too high. Nefarious individuals know this and deliberately copy this imaging technique as a way of selling overgraded notes. As such, it is up to every person viewing an image to use rough justice in estimating grade, and for that reason, I don't think it was unreasonable of someone to give an opinion of AU. It might even be generous because, as I said earlier, any unsoiled note in VF or higher could be made to look like yours in a small indistinct image. Therefore the offer of $70 doesn't seem outrageous. A decent-looking 1937 $20 note should be salable at that price, thus worth the gamble of such an offer.
BWJM made many good points about pricing in his editorial response of dealers versus private buyers, so I don't need to repeat them here.
The last point I want to add here is that many high-grade 1937 series notes are not in original condition. Many notes look like they were cleaned/washed/pressed/trimmed (or any combination of these), hence the risk factor of buying such a note makes it hard to offer good money without seeing proof that the note is original. Images where the contrast and/or brightness is too high makes a note look "flat", and flatness equates to being processed. You want people to see texture in an image, even if that means making the note's faults that much more apparent. If a note has good texture, it is probably not processed, and you should get decent offers.