I find the dimension very curious, they don’t fit any cartage exactly but here are two (2) other possibilities besides 8x60.

a) 8x56 mannlicher schoenauer see link below:
https://www.bing.com/images/search?view=...;vt=0&eim=1,2,6

b) .318 Westley Richards see link below:
https://en.wikipedia.org/wiki/.318_Westley_Richards

I suggest you re measure the casting very carefully, there may be some errors in the measurements.
Hope this helps.