I always found it funny that there was a mono pressing of DoFP at all considering that the album was recorded purely as a demonstration record to demonstrate stereo.
Not many mono pressings were made in the USA, since the labels were (mostly slowly but steadily) reducing mono to where, by 1969, everything would be 'single inventory' stereo (or whatever). But in England, where mono would be more commonplace for a few more years, a mono pressing was made, and remains easier to find than the US edition.
The only reason it got released was because Decca had spent so much money on the recording process, they decided to release it to recoup some of their cost.
It helped that "Nights" became a hit before the album got full release. And though it was only a minor seller back in 1968, the single did help make DOFP a sleeper success in the US, and over the next few years, as the band toured, it sold quite well.
I've never heard for certain whether the mono was just a fold-down or if it was a genuine mono mix, but I'd have a VERY hard time believing they'd mix that album to mono from the multi-tracks, since it was recorded specifically for stereo. So I'm sure it's fold down. A US pressing is going to be pretty hard to find because that album did absolutely nothing in the US until 1972. By then, the mono album was long out of print. As a rule, mono records were not discontinued to the general public in the US until 1968. Since DoFP came out in 1967, there is almost definitely a US mono pressing. I'm sure when one comes up on eBay, it'll fetch major amounts of cash.
Actually it was 1968 for the US album, and yes, it's a fold down, as are the next three UK mono editions. Singles were sometimes dedicated mixes, though.
From 1968 onward, mono LPs may have been pressed, but they were promo only. Case in point: Frank Zappa's We're Only in It for the Money and The Velvet Underground's White Light/White Heat, both released in 1968 had unique mono mixes and mono LPs were pressed, but they were promo only. There were never stock US mono copies that you could buy in stores. Another example was The Beatles' White Album, released in 1968. There is a mono mix, but it was never released in the US, only in the UK.
Actually some stock copies of both titles exist, but are few and far between. As for promos, once stock mono pressings were eliminated, promo mono's (on almost any label) were usually folds, not dedicated mixes. But it wasn't until 1969 that stock mono was all but gone in the US; there were quite a lot in 1968, just in reduced numbers. As for the WHITE ALBUM, the UK mono press was a strong selling import title for years since it was indeed unavailable here.
ED