Bottled gas lines

[Spectric]

The top half of the grate thing is solid over the exhaust with only 8mm clearance between it and the pipe. Presumably it's purpose is to divert the exhaust gas out through the slots on either side,

Going by the link provided by @toolsntat it is removed when running the burner on GAS, plenty of info on this cover and it's removal online.

https://www.motorhomefun.co.uk/forum/threads/water-heater-vent-cover.144507/

https://forums.practicalcaravan.com/threads/truma-water-heater-external-cover.15574/

Any heater that burns fuel and produces CO has been designed to be safe, these devices must not be modified because they then will no longer comply with the certification and can become dangerous so if you have one that is not functioning then there is a problem that requires the right person to rectify and do not start modifying the device because you think the design is wrong.