Its been a long time - I donated my 3455A - but I believe it had AC and DC coupling that could be selected in the AC mode. A real True RMS meter will read DC on the AC input. I think it had a switch that allowed this.
On one of my hand held meters, a Beckman true RMS unit, this is also a check for the True RMS converter, to apply DC instead on AC and verify it reads the same.
I think the 3456A was a close made unit to the 3455A, which I ended up with several of them.
Check the manual on the 3455A and see what the coupling options are on the AC mode.