When you are working with Honeywell Actuators, it is a good idea to familiarize yourself with their part number nomenclature. The attached pdf shows the complete Modutrol 4 model number breakdown. Here are some highlights: After the M (which stands for Motor), the next two digits represent the control signal. Common Choices:41 = two position line voltage61 or 62 = Floating72 = 2-10 or 4-20mA7x = other voltage modulating signals81 = two position low voltage9x = 135 ohm The next two digits after that indicate number of shafts, torque, and whether it is spring return or not. The chart goes into further detail, but the the higher the first number the higher the torque. If the second number is 1 or 4, it is non-spring return. Next up is the letter. There are a ton of options listed, but for our purposes, you only need to know 4 basic ones for most applications:A = no aux switchesB = 1 aux switchC = 2 aux switchesD = no aux switches, but likely has the cams to allow adding switches later. Must be a tradeline model. The last four numbers are specification numbers and generally do not specify function. To know what they mean, you would need to look in the catalog. You can also apply this knowledge to other Honeywell products Honeywell uses a similar format on other controls as well. Usually the first number after the letter indicates a basic function:4 = Line voltage6 = Floating or SPDT7 = Modulating8 = Low voltage9 = 135 ohm Knowing the above, you can look at a TB7980 or TB6980 thermostat and have a pretty good idea what it does. (TB7980 is 2-10 and TB6980 is floating)What about MN7220A2007? The 7 is a dead giveaway that it is modulating with either 2-10 or 4-20mA (2-10 in this case)ML7984A4009 - again, this is a modulating actuatorML6161A2009 - Floating actuatorT991A1004 - 135 ohm temperature controlT451A3005 - Line voltage thermostat While it doesn't tell you everything you need to know, it is a quick way to see if you are on the right track.