John T. Martin was but one Federal contractor during the war. There were several depots that manufactured clothing, along with hundreds of private contractors to keep up with the demands for several million fatigue blouses (the period term for a Union sack coat). Looking into the contract dates for J.T. Martin, his contracts for the majority of his coats were from 63-65, although he was prolific enough to have contracts from 62 through the war (just saying the majority of his coats were made mid to late war by numbers). The reason why so many vendors today utilize the J.T. Martin blouse to replicate is because so many of them are around to study when in comparison to the other places. There were thousands left over as surplus at the end of the war, and quite a few people and museums own originals to study in depth (I've viewed a few myself in the last few years). Another theory I have about JT Martins being copied so much is they have quite a few "cool" stamps in them. Size, contractor, and sometimes contract date marks are stamped into them, and people who like details today like having copies with all the details as well. In short, there is not really "one" blouse that can get you generically through the whole war, but most blouses shared some common traits. A well made, well researched coat, using the proper pattern, material, and construction is just part of the equation today. Although people won't kick you out of an event for having a late war blouse at an early war event, its always good to know the differences. My personal favorite for getting through a number of war years is an SA blouse (Schuylkil Arsenal or Philadelphia). They by and large were completely handsewn for most of the war, you can get a great kit for them from Wambaugh, White, and CO., and are great to learn to sew with their directions. If you don't want to sew, there are some great vendors out there with blouses to include Wambaugh and White, NJ Sekela, Carter and Jasper, Andrew Kasmar, Camp Randall Quartermaster, CJ Daley, and a few more if you look into it.