The DLL file in question is
omplrlib.dll and Helm has it’s own copy in
C:\Program Files(x86)\Rascular\Helm Designer.
Helm installs the latest available version of the DLL if none is currently installed. Helm V4.5.6 installs Harmonic’s 8.3.0 DLL.
You can replace Helm’s DLL with an older version, available from Harmonic. Harmonic should be able to advise you of the correct version for your Spectrum system, but Helm should be compatible with most versions.
Note that 64-bit DLL versions are not compatible with Helm.