Thank you both for your thoughts - I was pleased to be approximately correct!
Yes, Gibson did use rounded corner CdV cards at times in the 1860's.
This is the only Gibson I've got (so far) which appears to have some sort of 'landscape' as a background.
My Jayne Shrimpton book suggested that the use of the 'oval medallion' as in this CdV began in the late 1860's.
I aquired this CdV in the same batch as the one at the start of this thread (thank you ebay

) also in an oval and on the identical cardstock. To me it looks like mid-1860's - would you agree?