Signed Fan Cel from Arslan Senki
Here is a close up of the signature on this fan cel.
This fan cel was created by an American fan cel artist. At one time there was a webpage where many English-speaking fan artists would get together to exchange techniques, critique each other, and offer support. As selling on ebay became common, problems came up where some of their previous work was being resold as production material. There were even fan cel artists who intentionally tried to fool new cel collectors by creating their own “production cels”. This early group of fan artists eventually developed the policy of signing their work. These honest fan artists wanted to separate themselves from the dishonest ones. Since that original webpage has fallen into disuse and I do not buy a lot of fan artwork, I am not sure if this practice is still as common as it once was.
On an unrelated note, it was around this same time that the enforcement of copyright laws was increased here in the United States to include internet transactions. This led a number of fan artists getting contacted by lawyers/copyright holders with cease and desist orders whenever they used American based shows for their artwork. Understandably spooked by this, most artists shifted to only doing Japanese shows where language and distance offered better protection.