  We re-analyze published proton beam dump data taken at the U70 accelerator at
IHEP Serpukhov with the $\nu$-calorimeter I experiment in 1989 to set
mass-coupling limits for dark gauge forces. The corresponding data have been
used for axion and light Higgs particle searches in Refs.
\cite{Blumlein:1990ay,Blumlein:1991xh} before. More recently, limits on dark
gauge forces have been derived from this data set, considering a dark photon
production from $\pi^0$-decay \cite{Blumlein:2011mv}. Here we determine
extended mass and coupling exclusion bounds for dark gauge bosons ranging to
masses $m_{\gamma'}$ of 624 MeV at admixture parameters $\varepsilon \simeq
10^{-6}$ considering high-energy Bremsstrahlung of the $U$-boson off the
initial proton beam and different detection mechanisms.
