The central dirt tube in the cyclone bin often got blocked, as people only pulled the trigger and let the outer bin empty, without checking the central tube, where dirt would get compacted and stuck together - thus not emptying when the bottom flap was dropped. Eventually this central tube would get so full it would stop the dirt from exiting the cyclone cones, and block up the pre-motor filter constantly. People would not realise why the filter was getting clogged so quickly, and often throw the cleaner out, thinking it faulty.
Thinking about it, I'm not sure whether the very early models had that black seal on them, it may have been fitted as a modification at a later date, possibly if the post motor filter was getting blocked by dirt escaping the pre-motor filter up the relief valve tube. Don't hold me to that though. I've had a few DC07's where that seal was missing, and they are very cheap to buy on EBay.
Whether the seal is fitted or not, it makes no difference to the pressure at which the relief valve opens.