Anyway, on the double line precancels... they are known as DLE's which is short for double line electrotypes and named after the type of device by which the precancel was made. It's a very popular branch of US precancel collecting, and if I recall correctly, there is a catalog available just for this type of precancel.
Yes, there is a Precancel Stamp Society catalogue for DLEs. It lists the value for each different stamp showing a particular precancel style, which makes it rare among Town & Type precancels. The catalogue for Bureau precancels (stamps precanceled at the Bureau of Engraving & Printing in Washington, DC) also lists each combination of stamp design and precancel type, but that's not the case for stamps precancelled locally (except for DLEs and a few other specialized types).
Stamps precancelled locally are known as Town & Type precancels, and DLEs fall under that category. The Town & Type precancel catalogue lists only the combination of town name and precancel style - it doesn't attempt to list every different design of stamp showing each style of precancel. Good thing, as that would be a massive number. It's estimated that there are perhaps as many as 2,000,000 different Town & Type precancels when you look at all the different town name / precancel style / stamp design combinations. (Under 9500 different Bureau precancel stamps, though.)
The precancelling device for DLEs (something known as an electroplate that precancels 100 stamps at a time) is furnished by the US Post Office Department, which makes them kind of like Bureaus. But the stamps are not precancelled by the Bureau in Washington, DC - the electroplates are delivered to the individual towns and the stamps are precancelled locally, which is what ultimately classifies them as Town & Type precancels. They're kind of limited in scope because of that, as only 127 different towns were provided with these DLE plates, over a period of only 3 years. In total, it adds up to just under 12000 different stamps, when the inverts / double prints / other errors are included.
As to the Philadelphia precancel example posted earlier, that one isn't a DLE. It's listed in the Town & Type catalogue as precancel style L-1 E Var 2. The "E" means that the precancelling device is an electroplate, and it does obviously have two lines above & below the town name, but the "L" means the town got that device locally instead of through the US Post Office Department. Further, the number "1" means it was first of these local devices obtained post 1913, which is when the option of locally obtaining precancel devices was authorized by the US Postal Service (albeit a variant of the initial design, thus the "Var 2").
Way back at the start of this thread, the address for the Precancel Stamp Society was posted. It's definitely worth checking out for those interested in precancels.http://www.precancels.com/index.html
Precancels can be both endlessly entertaining and endlessly frustrating. Sometimes they're worse than Indian states for trying to figure out exactly what it is that you have sitting in front of you!