273 Harmonic Balancers

From what I've seen and done myself, all the internally balanced small block balancers will work on the engine (and not cause a bad vibration) with the following caveats. '69 and earlier have the timing slot cut for a passenger side timing marker and have one pulley hole staggered (it can be oblonged to fit the later pulleys however, and vice versa). The '70-2 have the timing slot on the driver's side and also have the staggered pulley pattern. The '73 and up balancers have an even bolt pattern where the pulley can mount 6 different ways. Now, I have seen different appearances on them through the years. some outer rings are squared off, flat and have even surfaces both front and back. Others have a half round groove cast into them for whatever reason. And when it comes to using the poly 318 balancers, they had both a manual trans part number and automatic part number. I don't know why and have swapped them without any ill effects. "66-9 balancers use the same 2658458 part number for 273-318-340 engines except for the '66-7 poly 318 with an automatic trans. which is 2658459. The '64-5 273 has a part number of 1947235 (both automatic and manual trans) like the early poly 318's with a manual trans. where the early poly 318 automatics use part number 2402090. Very strange.