well my expiriance is that the early Monacos seals / Rubbers changes in their chemical subtances and go sticky, oozing, and do a very bad oily mess to the glas / DIAL and case even it runs into the movement under the dial!!
Meanly the seals on monacos before 1973 had this fault and later the seals where made from a other material !So the mostly serviced monacos from Heuer had the later better seals inside, if a monaco is NOS but not worn( in a safe over years) , perhaps the ultra violet light couldnot hurt the rubber too much!!

Anyway i would open the Monaco give it a new service and change the seal a.s.a.p.! if the new owner doensnot do this the old seal will kill his fantastic dial and stick up the case sooner or later!
If anybody needs the spare seal-part or a new glas, we have both on stock!

greetings from germany

www.classicheuer.de Jasper Bitter