  This Letter reports one of the most precise measurements to date of the
antineutrino spectrum from a purely U235-fueled reactor, made with the final
dataset from the PROSPECT-I detector at the High Flux Isotope Reactor. By
extracting information from previously unused detector segments, this analysis
effectively doubles the statistics of the previous PROSPECT measurement. The
reconstructed energy spectrum is unfolded into antineutrino energy and compared
with both the Huber-Mueller model and a spectrum from a commercial reactor
burning multiple fuel isotopes. A local excess over the model is observed in
the 5MeV to 7MeV energy region. Comparison of the PROSPECT results with those
from commercial reactors provides new constraints on the origin of this excess,
disfavoring at 2.2 and 3.2 standard deviations the hypotheses that
antineutrinos from U235 are solely responsible and non-contributors to the
excess observed at commercial reactors respectively.
