A Contra Tenor (or counter-tenor) would be a male singing voice whose vocal range is literally contra-tenor - higher than the normal range, in comparison with Bass being lower. No doubt music buffs can supply better classical examples, but my first thought would be Barry Gibb's harmonies as part of the Bee Gees. Trebles would most probably be boy singers, or teens with high voices, thus giving an indication of the ages of those so named.

It must have been quite something when music at church was made by the people, rather than by one individual playing an organ - something for them all to be proud of taking part in.
Are your ancestors amongst them?