Correct bulb type oil pressure sending unit?

Well, is anyone interested in researching the bulb type oil pressure sending units. To start this discussion it seems like "way back when" Ford used to service this bulb with a couple of different parts, and that one had a "40 " stamped on the base while the second unit had an " 80 " stamped on it. I think maybe the 390 engines took the "80" and the small blocks used the 40, but I have no basis in fact for this. I once put one with the "80" in a 66 289 and although it read within the range of the gauge, it was on the low side, going about one third of the way rather than letting the needle stay in the middle or slightly to the right. I replaced it with a bulb that had the "40" stamped on it and all was well, with the needle going about two thirds of the way to the high end of the operating range when cold and remaining in the center when at normal operating temps. So does anyone have any insight on how present day bulbs are calibrated and can anyone list a pressure/ needle position relationship using common replacement sending units. I suspect that these present bulbs are calibrated to function with various pressures and may not even have a meaningful relationship with our gauges. Anyone worked on this?