So plywood cores would most likely be pine, or if imported who-knows-what? The weight of the cores seemed to vary.
I would not recommend anyone else using them for a project like this because of how they warp and bend a bit with time, I also had to sort through a pile of them to find any that were straight to begin with. I think they were two-dollars each, where quality four-by-fours would have sent the cost of the project into the stratosphere. But it was a prototype to see how my idea would work and it did it's job.
My other idea that I did not try was to keep one of the legs full-length and attach something to it so that it could be used to lift logs with a block&tackle off the ground and onto the buck if they were too heavy for someone to place there.
I weigh over 220 pounds and when I got on this buck and jumped up and down it showed no sign of flexing at all, nor did it ever with any log throw on it. An architect-engineer looked at it and said the design would hold the weight of an automobile as long as it was made carefully with all of the joints tight, but the tighter the joints are the harder it is to take-down and put back together. The pieces do have to be marked with hash-marks by the joints or in some other way because they like to go together in the exact way they were cut to mate with one another, which is another annoying feature that more careful construction might eliminate....
I very quickly laid the joints out with a 30/60 drafting triangle and got the thing up as quickly as I could.
Also skip the coal-tar and find something else. I found a metal container of coal-tar at a garage-sale for a dollar so I bought it, but it is very smelly and dirty stuff and it gets on your hand, clothes and everything else and is hard to clean up. Maybe they do not eve sell it anymore....