Yes- Eldredge and National are the same, The original company name was "Eldredge" and they merged with June and I believe one other, and the new company formed was named "National" however, they kept using the name "Eldredge" along with about 600 other names. National is one of the companies that would "badge" their machines with just about any name the buyer wanted. However Eldredge is a National company name, not a "badge" for a reseller.
The earliest were made before the merge, and are badged Eldredge, however Natioanl used the name at later dates, so having one with the Eldredge name does not indicate it's older. Mine is Eldredge and was bought new in the 30's (I have the original documentation).
The first use a different style of can, it's got "wings" on it. I acquired one like that, but it does not fit my machine. the later two styles don't have those wings and are very similar, and I believe are interchangeable.