This is a scan of a cross section (yes, I cut it in half!) of the typical 1 inch pulley used in Gilbert-Meccano outfits. The boss was mounted through the hole. I added the light lines because it looks very strange with no connection of the actual wheel plate. There is some remarkable forming involved here - I have used the word 'excruciating'. Note that the pulley is formed from a single plate. I think I have seen these in Erector sets, too, but I don't know the Erector number, if there was one. These were listed as part number 22, and replaced the typical Meccano version.