  We investigated the MeV properties of 173 unidentified or only tentatively
identified EGRET sources listed in the third EGRET catalogue, by analyzing the
simultaneously collected COMPTEL MeV data for each individual source. The
sources can generally be divided into 4 groups. In this paper we focus on one
of these, a group of 22 EGRET sources for which we can provide additional
constraining information: their spectral extrapolations from the energy range
above 100 MeV towards lower energies overshoot the fluxes or upper limits
derived simultaneously at MeV energies. This means that for these sources a
spectral turnover/break between 1 MeV and 100 MeV is required. At least two of
these sources, but most likely the majority of this sample, have the maxima of
their gamma-ray luminosities in this energy band. The sources have rather soft
EGRET spectra (average photon index: 2.72), and seem to spatially cluster in
the inner Galaxy. Variability analyses revealed 11 out of the 22 sources to be
significantly variable. Object classes proposed as possible counterparts for
the unidentified EGRET sources are discussed in the light of these additional
constraints.
