The term "Modern Cut" is a misnomer. The correct terminology is round brilliant cut ( most likely ). Round brilliant style cutting is different. The old european cut diamonds had higher crowns, smaller tables, and larger culets than the current cut stones.
In the past 20 years much has been done in improving gemological diamond grading.
You say you're 'looking' at this diamond... Does this mean you're considering buying it with a 20 year old report?
It needs to be updated, and if it doesn't have a major lab report, I would demand this be done, so you know the grading is reasonably correct.
Also consider that if this diamond is used, that you compare pricing against other used diamonds, rather than new diamonds. Used diamonds can have wear on them, i.e. nicks, chips, scratches etc.
Rockdoc