"The United UWB-2 wallbox is probably the rarest wallbox that we actually know was made and sold commercially.
In 1990 I bought 22 United wallboxes - 17 of them were model UWB-1 and 5 of them were the extremely rare model UWB-2.
The serial numbers for the 5 UWB-2's were 1000, 1002, 1003, 1004 and 1005. I know that #1028 sold on eBay in 2002.  Those 6 wallboxes are the only UWB-2's I have ever gotten confirmation on,  and I have asked a lot of advanced collectors about them.
I strongly believe that the wallbox. serial # 1000, is the first one shipped commercially."