From what I can see, with a cursory visit to google "red dot HP 35" and my prototype 'brown' red-dot case (see images):
1. red dot hole correctly placed inside and out plus
it appears to be the correct diameter.
2. serial number falls in the correct range ~< 4000
3. correct silk screened bezel
Also look at this old thread:
red dot talk from 2000
The upshot is, by looking at the inside back where the hole should line up with the frame work. I have seen a fake where the hole is WAY to big and not centred on the switch opening or on the reverse, inside the rigid molded supports.
Would be nice to see the label at the bottom end of the calculator. It should read only "HEWLETT PACKARD" without the 35 added. This is not always a perfect check either, as it was HP's policy to update the label if the original was faded or damaged with a new one in stock on a warranteed repair. They didn't care what was applied as long as it was correct to the calculator. Hence a number of HP 25's running around with HP 25C labels; HP 41 Cs with labels mixed with CV and CX. Got this from an HP repair centre employee.
Cheer, Geoff
Edited: 15 Dec 2010, 2:30 p.m.