Pete,
You might check with a guy named Henry Brubaker at hcbrubgmail.com; Henry has a good number of used Savage pistol parts (all three models, but few m1915 parts) he is selling; some of these are currently on GunBroker. You can tell him I gave you his email.
With a s/n of 57200, your pistol is from the last version of the "early" model 1907, the 5th .32 m1907 version, what is usually called a 1907-10-2, made 1910-1911, approx s/n range 19500-65000. The parts diagrams for the model 1907 & 1910(!) Savage pistols on Numrich are for your pistol. (see:
www.gunpartscorp.com/catalog/Products.aspx?catid=1334)
The breechplugs for 20861 and 53283 should work -- these pistols are the same version as 57200, and use the same version of the breechblock. In fact, while there are five different versions of the .32 breechblock for the m1907, m1915 and m1917, all five of the early m1907 versions use the first version, which is the only one with a single lug.
On the other hand, it is not unusual that some (usually minor) fitting might be required. While these pistols were factory made using standardized parts, 100 years ago "standardized" meant a lesser degree of uniformity than now. Breechblocks were individually fitted to slides, and the rear of a slide could be modified to fit a breechblock (which is why some Savage pistols have a greater or lesser space behind the rear sight than other examples of the same version).
BTW, while the extractor did evolve, my best recollection is that you should be able to use any model and version's extractor & spring in your pistol.
Bill