Pete I think the date stamped on the various pipes is the date of manufacture, they then have a five year lifespan from that date.( Shame really as my pipes look like they were only fitted yesterday)!! But rules is rules I guess!
The mind boggles how they work out the ventilation lark, I have those UFO type vents on my boat but also the gap at the front and back of the sunroof are taken into consideration and there is a small sign warning not to block these areas from her hireboat days..