pardon but i am not familiar with bus models.. but merc 0405 is so
smrt-ish
http://webzoom.freewebs.com/busireland/2100%20GVS%2024.01.07.JPG u
sure 155 uses it?
There are different bodyworks for buses that can be used.
Walter Alexander bodies and Duple Metsec bodies are
the ones that SBST uses and SMRTB uses mostly the
Hispano-bodied ones and a few Volgren-bodies buses for the O405s.
Just to show you all the bodyworks:
Walter Alexander Bodywork (SBST):

Duple Metsec Bodywork (SBST):

Hispano (SMRTB):

Volgren (SMRTB):

Of course the O405s aren't the only buses which have different
bodyworks although they share the same chassis. There are others
too (E.g. Scania L113CRL, Dennis Lance, Volvo B10Ms etc ...
...)