Mark IIb effects loop question

[Murfman]

I sent my boogie into Mesa to get a tune up and to get the upgraded effects loop circuit. This was done about 4 or 5 years ago. Can someone tell me if this is a series loop or a parallel loop. I'm trying to figure this out. I would love some input on the differences between these types of effects loops as well - are there benefits to one over the other?

Thanks for any input you can send my way-
-Murf