Historical field guidance treats Digital UNIX releases in the V3.2x family as predating DE500 support and therefore uses the DE435 as the conservative adapter choice.  The Avanti 5.1.0 evidence does not qualify every V3.2x kit or establish that every such kit rejects a DE500, so do not treat this as a newly tested release-by-release boundary.

Use the DE435 unless the installed release's own driver documentation and a disposable traffic test establish another supported configuration.  Verify that the expected tu interface is present and that traffic works in both directions before relying on the configuration.