this is harder than it might seem. The quantity of gas under the can will be very small. If you suspect that it might be air then the vacuum will have to be high prior to opening the can. Even a 1liter chamber may have 10,000 more volume than under the can and to observe a spectrum you will have to end up with a pressure in the mm range.
What if it turned out to be, let's say argon. Does this mean that argon is necessary? It might just be a convenient, clean gas for back filling the diode. Commercial manufacturers fill tires with nitrogen and windows with argon. These fills are not necessary, but may give some advantage to the product that in the case of the diode would not warrant the effort.
No matter what the gas, if it was anything other than clean air then these harvested diodes will simply not last and they will not be worth much. It is unlikely anyone here will be able to duplicate the manufacturer's can placement and back fill with a special gas.
Rather than try to analyze the gas I would test the un-caned diode to see if it lasts without the can. If it does then that might be enough. If it doesn't then it might still be worth trying to correct the beam out of the stock can.