They look similar externally and that's about it. A more accurate description would be 273/318 are basically the same and 340/360 are basically the same and the late external balanced 340 morphed into the 360. The 340 HiPo evolved from the 273 Commando not the 318 . There's alot of 318 is the same as 340 internet BS its just an inaccurate claim.