Bales or other bundles of something, but not peat. THe compost was freshly composted horse manure and straw, done on site. My guess is we will never know unless we get lucky with an old photo, or by some lucky chance someone who knows about traditional mushroom growing sees this. Now this text in on the web, search engines will pick it up.
We have one or two other mystery objects to wonder about. I'll try to find some photos.
Anyone else got something?