Yes, it is a personalised plate. Regular Nova Gradiška plates have only three digits. You can either choose a personalised digit and letter combination in the standard format like this plate (cheaper) or a complete personalised plate (more expensive).
piggy Where I can check the insurance for this car?
The insurance database can be found on the website of the Croatian Insurance Bureau / Hrvatski ured za osiguranje on the following link in English: https://huo.hr/en/insurance-enquiry You only need to enter a date and the plate of the car. The plate needs to be entered without spacing or dashes, in this case: NG0001SM
piggy Where I can check the insurance for this car?
There is currently no insurance for this plate in the insurance database. It looks like this plate has been deregistered since the picture was taken.
"The Information centre has no record of a valid Motor Third Party Liability Insurance Policy for the vehicle registered NG0001SM on the entered date of accident: 26.10.2022."