Are you referring to an Inspiron 3668 Desktop that shipped 7 years/4 months/18 days ago on March 6, 2018?

@DELL-Chris M​ 

Yes.  And for 7 years Dell diagnosics has been telling me it is a Mobile Venue Pro in the US.  Even if I write in the correct service tag, the system comes back with information on this laptop

The Dell UK Support site correctly identifies the private Service Tag stating with a "G" as an Inspiron 3668 Desktop. As to why the Dell software does not, unknown,
 

In my opinion, I would not use the online "Troubleshooting and Diagnostics". If the Inspiron 3668 Desktop still only has 8GB ram, I would also not be using SupportAssist. In fact, I would uninstall most of the Dell software and run a "cleaner" operating system.

I would instead only use the pre-Windows F12 Diagnostics to check the hardware.

@DELL-Chris M​ Thanks for your comments. 

My PC has had 16Gb RAM installed very soon after I bought it.  The original motherboard was replaced as well as it was faulty and fried the first set of replacement RAM.  I think that is when the misidentification began.  Dell and Microsoft between them has, since then, seemed to conspire to create all kinds of issues.  And, as I said, Dell just stopped talking to me well before the warranty expired.  It has been impossible to contact them since.  My confidence in Dell is not high.

Interestingly and amusingly, when I clicked of the F12 Diagnostics links, a helpful note appeared: "This article does not apply to Mobile Venue Pro".