Lugwork, in general, but the seat cluster in particular, is one of the best ways for framebuilders to distinguish their work. As you can see from all the images posted to this thread, there is quite a bit of variety in fastback seatstay treatment.

Here are a number of shots of fastback seat clusters on some of my bikes. Here's my RRB's cluster:

A Trek 770 I used to own (Trek did a lot with these "pluggable" seat lugs to simplify construction - I think [MENTION=104946]b dub[/MENTION] has it now):

A Serotta Coeur d'Acier which passed through my hands a couple years ago:

My Redcay:

My Atala Super Pro: