Fantastic!  
The search is going to be well worth the effort no matter what you decide to buy.

Have fun, Marine!

Slowly but surely you will close the loop.  In the meantime, consider a Marathon MSAR or GSAR with the Globe and Anchor on the dial For your dad.

 Marathon is a no foolin’ provider to the US military and USG, the “SAR” in the title stands for search and rescue.  Better still, the quartz models won’t break your bank account and are available now.  I have the Arctic MSAR quartz and a “sterile” Marathon GP (general purpose).  Both great watches with fantastic tritium lume.

Don't give up the chase for that 1970 classic, but in the meantime, you could honour your dad with the current generation of CSAR watches.  
 

my two cents.

JR

Back in the age of steam (and T-10 parachutes), folks wore their watches face in because acrylic dials got really scratched if worn “normally.”  Also, a watch with bright lume was a dead (literally dead) give away if you are trying not to be seen.   

Now that the G-shock and Timex Ironman are ubiquitous downrange, I would guess it’s just preference.  I wear my G-shock the standard way now (of course, I’m a pensioner now…).

Brother,

it is an incredible story and while your Pop is still going strong, I think you should do an oral history.   i was too young when my dad (USN tin can sailor from WWII) died to ask the right questions and I regret it to this day.  Any/all details are well worth capturing and, assuming your dad agrees, sharing with vets either in a formal publication or something like medium.com.

When you do ask about his service, you can drill down on whether it was a PX watch or some sort of issued watch.  If the former, then you can do plenty of searches on PX Seikos of the time.   It could be either of the watches you mentioned or something entirely different! 

If an issued watch, then as mentioned by a previous writer, his teammates should be helpful.   I would be surprised if it was an “issued” Seiko.  At the time, issued watches were Benrus, Waltham and Bulova.  MKII watches make a terrific Benrus homage (though you have to be lucky to be ready when their small batches are for sale - so far, no luck at this end).

anyhow, it is a great project and I wish you good hunting and good fortune!

JR