The job at the end of March was an upgrade. A tachometer off a turbo came up on ebay and I managed to win the bid with a few seconds left. Anyway, unlike my existing tacho, this version has the OBC display. Whipping the old tacho out was a 15 minute job and the new one slotted and plugged straight in (the wiring harness already had the alternative plug and I already had the control stalk from replacing the indicator stalk unit).
The 964 OBC is very basic compared to newer Porsches but at least I now have a very useful digital speed display in the tacho. However, it was only once I turned it on that it was a KPH version (they don't in built switch-ability) but it's still very useful since the speedometer is always obscured by the steering wheel once over 70 mph!