As explained by Chief Norman and others in the Gettin Salty episodes, the current collapse rescue units are the product of an evolution of SOC. Pre-911 and before the formalization of SOC, each of the five rescues had their own speciality. Rescue 1 (and 5, I think?) had the specialty of dive rescue, Rescue 4 had the speciality of HAZMAT (before HM1 was organized in 1984 then continued as a support function after that I believe). Rescue 3's speciality was collapse. R3 with Collapse Rescue 3 responded city-wide on collapses. From Signal's post above, it looks like this was true from 1988 to 2005. With the massive expansion of SOC pre and post 9/11, individual rescue companies no longer had their own specialties, but rather all rescues were trained and equipped with all specialties. The idea was that each borough should be able to operate independently. Thus, all five rescues got dive trained, all five rescues got collapse pieces, etc. This is to say nothing of all the other aspects of building out SOC such as the return of Squads, HMTUs, SOC battalion evolution into RB, MB, HMB, evolution of the Marine division, CTU, etc. This is my understanding of things based off listening to discussions from Chief Norman, Chief Salka, Chief Lafemina, Chief Richardson, and other great guests on Gettin Salty.