One of STLSoft's users is reporting weird behaviour in the Windows Registry Library, when used on 64-bit Windows (x64).
It looks like the call to winstl::dl_call is defective, since x64 does not support different calling conventions.
I must confess I'd overlooked this potential issue - in part because I've still not got around to fully implementing the STLSoft internal automated testing harness - but I would still assume that the code would work, because the calling convention information, if any, would simply be ignored.
We're still looking into it. I'll post more when I know more ...