Interesting. At sea level my ST 51 turned a 12x6 APC (and also 12x6 Graupner) at 10,000 on 5% nitro (although I always run a bit richer than some of my clubmates).
10x6 is NOT a normal size prop for a 51; too small, too nosiy.

For comparison - best prop for my ST51 in a 5lb Acrowot was an APC 11x6 but it was too nosiy to meet our noise limits. On the 11x6 verticals were effectively unlimited. On the 12x6 they were adequate.

Mike