What are the first two digits of the serial number? According to the IC-R8600 service manual, the USA version is 02, and a "USA-01" version is 04. For many years, Icom has put the version number in the serial numbers of its radios. A similar serial number scheme exists for the IC-R30 I have, based on whether the version is the "USA" version with the 800 MHz gaps or the version which doesn't have the gaps.
You can also try tuning the 800 MHz range, seeing if there are gaps around 824-849 MHz or 869-894 MHz. The "USA" version should not be able to tune those ranges. The "USA-01" version should be able to do that.
I got one of those Japanese manuals with it but haven't bothered to read it.
Fortunately, those gaps don't bother me.
I took a punt buying one from Japan, but there was not a lot to choose from, who would deliver to my location in NZ
My local supplier had some arriving in November, and they were another $1500.
The first challenge I had was changing it to English.