TLC - 11/04/2016 12:22 PM
In the link given by Phil, the SPG at the bottom shows a safety relief - a orange O’ring in this example. If you were to dig out that O’ring on that model it just has a series of small holes to vent off any excess gas in the unlikely event the mechanism fails. Opening your cylinder slowly would be wise option, and the hose is more likely to fail than the SPG.
The main face of an SPG is not pressurised. If the SPG has a vent, it will not come out like a bullet - that wouldn’t be an improvement in safety would it? You should keep your hand clear of the back, and in some case the top of the SPG, although the SPG will typically be in a housing.