Back in the early 80's, Boeing did some data gathering on contra-props with a Gannet (until they ran out of tyres) - they were doing the Unducted Fan (UDF) at the time. I was told by a denizen of their Advanced Project Office that the real advantage of the twin props was that it reduced losses due to swirl in the airflow - if memory serves saving up to 5%.
There's a Double Mamba with gearbox at the Fleet Air Arm Museum at Yeovilton - a check of their website might provide what you want. Otherwise try looking up TSAGI, the Russian agency that did the Bear prop developement.