I believe the 'standard' is one free return (with the girl's owner paying a nominal amount for the extra board) in the event of one or no live kitten being born. And the stud owners I've spoken go on a 'if she hurts him that's my problem, if he hurt her that's yours' sorta basis.

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Lily - silver Egyptian Mau
Jasper - Burmese X moggy - missing!
Electra - silver Egyptian Mau
(Truffles - chocolate tortie syrian hammy - bye little one, I miss you)
And 6 little balls of silver spotty fluff! OMG KITTENZ etc