Praenestine fibula

Hi, gang! I have been reading this list with enjoyment for awhile now.
I am a theoretical physicist by trade, with just an amateur interest
in historical Indo-European linguistics. I thought that I would break
my silence to ask the experts a few questions. Here is a simple one:

I was reading recently Andrew L. Sihler's "New Comparative Grammar
of Greek and Latin" (Oxford, 1995). However, I was surprised by his
assertion, made several times, that the famous archaic Latin inscription

Manios med fhefhaked Numasioi

on the Praenestine fibula is a 19th century forgery! This example is
always given as the earliest extant Latin, e.g. in Cyril's recent
on-line comparative Latin grammar. I was thus even more surprised
that Sihler did not give a published reference to support his assertion.
Does anyone have any information on this?