Sadly, PrimeGrid does not show this in either the computers table or the computer details. One or another project with old BOINC server versions don't show it either. I guess PrimeGrid's server version is not overly old, but quite customized, and this feature slipped through the cracks or was actively disabled.The command
gives a "model" name to the client instance. After you triggered a project update (or the client issued a scheduler request for its own reason), a new column with the heading "Model" appears in the "Computers on this account" table, showing the MODELNAME string which you set for this client instance.
boinccmd --host HOSTNAME:PORTNUMBER --passwd PASSWORD --set_host_info MODELNAME
In the individual host details page, this string is shown in a row with the heading "Product name".