When the HP-35 was turned on, a red spot showed through this hole. The serial number is printed on the back, between the two lower rubber feet, it is 1143A followed by the unit number stamped into the label in letters 1mm high. A batch of nearly identical HP-35s was made for a customer company who placed an order for them. Then the first HP-35s for general sale were made, still nearly the same, but with the unit number stamped in 3mm high digits. The next set of serial numbers began with 1230A - showing that serial numbers of that time did not follow production dates closely.
Later models have the other functions printed on the keys as well.
The function keys are horizontal rectangles on this version, not square buttons, and the trigonometric function keys are the same colour as the others. This later version is less common, though not as rare as the first one. Another bug is that the sine of some small angles comes out completely wrong - this was dealt with after the other bug, so later versions have neither bug. A after some model numbers that originally had no letter. This makes the model numbering scheme more uniform, but is not really necessary.