SEPTA consists of over 1400 vehicles, each of these buses/trolleys/trains may be specified the same on order but after a well in service, SEPTA decides to test new equipment on these units. The New equipment could be anywhere from new propulsion systems to a new designation signs. Although not all notible oddities can be placed on this page, they are mentioned in this thread.
http://philadelphiatransitforums.cjb.net/topic/7086217/1/
1981 Kawasaki B-IV DE #666
After many years in the shops, 666 came back into passenger service on Jan, 23, 2009. 666 became the first BSS train car to have an all silver (Naked) theme after the car was serverly tagged with graffitii. “Sources say that the car was so heavly tagged that removing the graffitii actually removed the orange trim”, Brandon S. 666 also has broken digital MPH readers in both of the cabs on each end and retains different font fleet numbers in red on side A of the car. A week after 666 returned to service it got an orange stripe under the roof lining impersonating the appearance of an M4 car. 666 looks similar to the CW cars but if you look closely, you will see the differences. During the SEPTA Rail Roadeo of 2010 it was reported that the rest of BSS cars will have all thier orange trims removed and keep a scheme like 666, therefore it would be easier for crews to remove the graffiti from trespassers.
Also take a look at this page for more info on the Modified BSS scheme.
http://www.philadelphiatransitvehicles.info/bslmodifiedlivery.php
666 at the 2010 Rail Roadeo. Picture Taken by Tritransit Area.