The burst disc is a safety valve that blows beyond specific pressures
These look like bolts in the sides of the cylinders regulator and will have a high pressure burst disc nearer to the cylinder and a low pressure nearer to the marker
They are 'fuses' for air pressure
Expect an 1800psi disc at the output side
5000psi at the cylinder side of a 3000psi cylinder
7500psi at the cylinder side of a 4500psi or 5000psi cylinder
These exceed the normal operating pressures, but are there to protect from excessive pressures or blockages
See the UKPSF HPA 1 form and other info on air:
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
See this video for a guide on replacing a burst disk and checking for regulator damage