My dad bought this watch for my brother as a gift in 1970.

Serial No. 1875419

Features:
• Plaque G 20 SAD
• Movement: Automatic
• Case: Stainless steel, 34.5mm diameter, 10.5mm thickness
• 14K yellow gold bezel and lug caps
• Dial: Gold batons and markers
• Band: 18mm aftermarket black leather strap
• Buckle: Yellow gold plated

My questions:
1. Does anyone know what caliber it is or do I have to have a watch maker open it and look inside?
2. Do you get to the inside of the watch from the front or the back?
3. Does anyone have more information about this model beside what I included above?
4. What is the average and maximum online selling price of this watch in the condition that this one is in? Is $450 a realistic number?

Thank you for your time and assistance.

- yahamshari

1) Zodiac 72.  It's either a normal 72 or 72B.  The 72B will stop if you pull the crown out.  The 72B hacks for better time setting and has a push-to-set date.

2) The back, it has a snap back cover most likely, any watchmaker can open it up.

3) In 1970 Zodiac was changing up its lines quite a bit and experimenting so not all watches belonged to a specific family.  However everything was resolved a few years later.  This would have fallen into the Hermetic family if it were a couple years younger (or older since the Hermetic line has been around for a while).

4) $450 is realistic if the watch has been serviced and is in excellent shape.  We would need to see more pictures.  Your dial and crystal seem to be clean though I do see a bit of speckled patina on the dial near the date window.