And I even searched.
Can anyone shed some more light?
Three and a half minutes to ride a quarter mile? Sounds more like an endurance feat than one of speed. It’s not clear whether there is actually a seat on that thing, or if he just holds onto it and pedals. That would make the work more like riding with the seat out, which is a lot more strenuous. All the more reason to get through that quarter mile a little faster!
That distance is about 400 meters, and a good track unicycle time for that is around a minute. He had a smaller wheel, and possibly longer cranks as well, not to mention possibly not a seat.
It does look like it might be the back end of a penny farthing, though those were definitely outdated by that time period.
I could be wrong (and likely am) but in lieu of a seat it looks like he’s holding onto some sort of bent pole to support his weight. It also looks like the grass would cause pedal strike. 
Tho likely to be lying around somewhere, possibly to be cut-up by a unicyclist with a taste for the unusual?
OMG Herbert! thats my last name!!!
One minute around a 400 meter track is 14.8mph. But since you would not have a moving start, and depending on the size uni and cranks, it would also take a few seconds to get up to your max speed. So to make up for this lost time, you would need to hit speeds well in excess of 14.8mph to finish in 60 seconds.
It is a challenge for many 36er riders to maintain 15mph for even a 1/4 mile, on flat. To maintain more than 15mph on a smaller wheel for that distance, especially on a oval track with turns, 60 seconds would be a great time. I took me several seconds to get my 36er up to speed when I did my track run, and it took all I had to finish in 58 seconds! :o