This site has quite a bit of info
http://2spools.blogspot.com/
The first picture shows three machines of different ages - but they all look very similar on the outside. One thing to keep in mind, the very oldest will be Eldredge, however the very youngest can also be Eldredge (as mine is).
Some tidbits - this sight says all the machines have the same patent dates, early ones have a HXXXX serial number, mine is H61241. So if yours has a HXXXXX (five digit) number, it's not going to be from 1913, and if it's got a H5XXXX or later, it's one of the last made, in the 30's
I have a can for the oldest style, so I'm looking for a machine that it goes with. The most notable difference is that the slide plate does not "slide" but has a pivot point instead. So that's not too hard to look for in pictures.