Aaarggh. That may be unknowable. But based on the fact the company was Italian, I would guess ess-chee, even though ESCI is an acronym. swannymodels.com says:
"By the way, the name, “ESCI,” is actually an acronym, like RADAR or SCUBA. It is pronounced “esh-she,” although just what the letters E.S.C.I. stands for is still anybody‘s guess. If anyone knows, myself and a few other devotees would sure like to hear about it! (Editor's note; The acronym ESCI stands for Ente Scambi Coloniali Internazionali which translates as Company for International Colonial Trade.)"
This from Wikipdeia:
"This article is about the Italian model aircraft manufacturer...
ESCI was a well-known Italian model aircraft manufacturer. It is no longer in business. It specialised in static models.
They started manufacturing aircraft, military vehicle, car kits and figures around late '70s early '80s. Some of their kits were famous and also officially awarded (Model Fan magazine) for their accuracy and smoothness. They were pioneers of engraved panel lines at injection moulded kits alongside with Hasegawa."
http://en.wikipedia.org/wiki/ESCI