In the global section of smbd.conf, I've experimented with various values for fruit:model to see how the icon is displayed on macOS 10.14.3 (Mojave) clients.
These values displayed an icon specific to the model: Xserve, MacPro, iMac.
As a note, only the discontinued Mac Pro icon was displayed (the boxy one - not the cylindrical one).
These values displayed with an icon resembling an Apple Cinema Display: MacBook, MacMini, MacSamba, TimeCapsule, RackMac.
The generic icon was expected for MacSamba but not for the others.
Some unhelpful suggestions were to use values from /System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist to get specific icons.
While I didn't try every single model, most resulted in the generic icon.
Also, most people are confused about whether to specify the icon via Avahi or Samba. In fact, most examples provided show the smbd.conf setting the icon as an Xserve and the Avahi service as a MacPro.
Note: Avahi does not have issues with icon setting like Samba does.
Seems to be a duplicate of a fix we have already in master
*** This bug has been marked as a duplicate of bug 13746 ***