There IS, however, a folddown mono mix of DOFP. Supposedly there was a US pressing, but damned if I've ever seen one. I do have a UK pressing on the DSS Deram label(as well as LOST CHORD), but both are folds, so no biggie if you don't have them.
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.
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.
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.
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.