I think someone has used the 407 arc assembly on the 507s in the past successfully, as they found the 507 arc uncomfortable. The new type Lambda arc can't be used on the old Lambda/Lambda Pro/Lambda Signature/ Sigma/ Sigma Pro earspeaker cases, unless you remove the metal part that the old arc assembly used to insert into (i.e. the newer plastic pin has a greater diameter that the older metal pin).
All the new Lambda's share the same arc since the 202's which have a bigger diameter pin of the arc assembly that goes into the stator housing (unless there is one special off model that uses a complete different one). The new Lambda arcs/headband assembly can also be used on the vintage Lambda models, you need to use a pair of tweezers to unscrew the barrel secured into the pin area of the stator housing. Once removed it gives the right diameter clearance to install new Lambda arc/headband assemblies.