Seems like it could have been laid out fairly plainly from early on.

One block to someone who is "owed" a block for every "x" number of blocks produced, with the back ordered blocks being delivered in order of those who paid first, get theirs first.
Once the production levels had become somewhat known(x number of finished blocks being delivered per month), then a schedule for filling the back orders could be established, along with projected delivery dates for each of the people waiting.
Then it's just a matter of letting them know when their block is projected to be done.