Piettas take M6x0.75 thread nipples. The Treso 11-50-10 is that thread and (allegedly) takes No. 11 caps. However, I found CCI No. 11's were loose on them, while Remington No. 10's, Remington No. 11 and RSW No. 1075 seemed to fit ok.
You can get Treso 11-50-10 nipples at Track of the Wolf under part number PIR-A, or at the Possible Shop under part number 12-50-10.