This started with the beginning of the Incident Command System or ICS. The early trend was to have common reference. Some used engine... some pumper... then some used truck... some ladder. Some things just didn't fit. No problems when communicating with your own department... everybody knew what engine 1 was. Then a common radio frequency and a mult mutual aid response... the crews would be excited and not add the name... like Sylvania engine 1. They all would ID as engine 1... but who was it? Sylvania, Springfield, Ottawa Hills... nobody knew who was who! Another problem... some stations had more then one engine. For example... Sylvania station 1 has 2 engines. The first one was easy... engine 1. But what to call the second engine. Not engine 2 because that conflicted with station 2 engine 2. They started trying engine 1b and 2b... all kinds of stuff like that. What a mess! I liked the old way... 611, 612 and right down the line. By the way... that numbering all started back with the Lucas Co Fire Chiefs back in the 60's. Chief Elmer Cline came up with the idea. They tried to get Toledo to try it. I think they were assigned the ID of 3. So TFD station 7 engine would ID... 371. A ladder from TFD station 6 would be 367. But the problem was what to do for places like station 11, 23, 25, etc. Also the ICS trend was to use common terms in place of 10 codes. Signal 2 in Lucas Co was not the same as a signal 2 in Monroe Co. Back in the day... Lucas Co (except Toledo) had a common signal code system. IE: Signal 1 = enroute, signal 2 = on scene, 3 = in service, 4 = ok or yes, 5 = repeat, 6 = radio test and so on...