[Same disclaimer as before...]
Yes, I think it's a Perfect Passive Participle, used with est
as the perfect passive indicative, one way of translating which is '...has been...' - so 'has been written'. French does much the same, I think: '... dans laquelle est écrit SALVE.'
Also, apparently http://www.freewebs.com/gjcl/tutorial/participles.htm
o The perfect passive participle can be used to form an impersonal passive that removes all responsibility for the action. This is commonly used to describe the action of mobs, which have no distinguishable agent to be the subject of an active construction: Ventum est ad Forum turbae. The mob came to the Forum (It was come to the Forum by the mob).
which would seem to fit here (assuming the mob element isn't obligatory....