The seal looks fairly old, so may be original? ...and yes I have seen that sort of seal before...not sure if this was done at the EG factory though?...could be?...but then it could also be that the importer/retailer (Roberts) carried out some improvements to their range of these guns? ...as Lane brothers certainly did do to their "Jewel" Gem, touting them as being strengthened and improved in adverts of the time.
Anyway, if you want to replace it as it is?...looks like you'll have to cut the old seal off and try to slide the washer down the spigot then peen the front edges of the spigot back in to get the washer off, then go from there.