Help Identifying Vintage IWC – Possibly Ref. 810 with Cal. 89?

Hello everyone,

I recently acquired this vintage IWC wristwatch and was hoping to get your help confirming the reference number and movement.

Details:

• Case serial number: 2078636
• Movement marked: International Watch Co. Swiss
• Movement beats at 19,800 vph, runs very well (0s/d, 282°, 0.0ms beat error on Timegrapher)
• Caseback marked "International Watch Co. Swiss" and "Staybrite"
• Two-hand silver sunburst dial with baton markers
• Diameter approximately 34–35mm
• Manual wind

Based on my research, I believe it might be a Ref. 810 or R810A from around 1964, using the Caliber 89 manual movement.
But I’d really appreciate help from forum experts in confirming whether that is correct—or if it's a different reference or movement (perhaps Cal. 402/403?). The seller claims it is IWC 2400 but I honestly do think he might be wrong. 

Photos of the case, dial, movement, and disassembled parts are attached.

Thank you very much in advance!

Correction: Looks like movement is cal 403.

Hi Hoonypoon,

the case of your watch was produced in 1973 and most probably delivered in 1975. The reference nr. is 2400. And the movement most probably Cal 403. Your watch has definetely not a calibre 89 movement.

br Cromagnonman

Hi Hoonypoon

here a picture taken from the cataloge 1972 (thanks goes to cellar). It shows the Ref. 2406 with a different dial.

And another Ref 2400, this time with an automatic movement.

br Cromagnonman

Beautiful watch :o)