Hi Andy always willing to learn.
I personally am not sure if they were all medics in the pictures or just marks for all navy shore parties wither they be medics or whatever. Someone does say that shore parties had yellow circles. Again how can we ever prove it one way or another. It just may be me thinking alot of c**p as usual.
Here is a link to a thread on these which leads to another two links. WW2 US Pacific Theater Medical Officers Fixed Bale M1 Helmet - FS Helmets & Accessories - U.S. Militaria Forum
If you find out anything else please let me know.