While I really enjoy the "rhythm" of the timing you currently have the stamps set at, in reality they would have been more evenly spaced around each revolution so as to evenly load the power source driving them. Unless I'm not seeing correctly, it does look like you have all four stamps being lifted at the same time, although not at the same level of lift.
Wonderful that you have gotten a pair of them Bruce, as it was quite common for mills to set them up as individual banks within a larger series of banks. A twenty stamp mill might have four banks of five stamps each, and run all twenty much of the time, but each sub-bank would have individual in-feed chutes and out-feed paths so that any one set of stamps might be shut down for servicing, cleaning the tables or awaiting more ore, or perhaps some particular grade of ore that it's tables were specially adjusted for processing.