Everyone who ID this as a G-20 class Patrol Vessel of the Mexican Navy are right. Thanks. There were ten (G-20 through G-29) of these units built in 1934-5 in Spain (to a British design), with half assigned to the Atlantic coast and the other half assigned to the Pacific coast. Which specific unit this is, is unknown.
I found the vessels listed in my 1941 copy of Jane's with a small but fairly nice image of G-21. These vessels were rated at 130-tons (standard), 153-ft at the waterline, and 16.5-ft wide. The various online references have varied armaments that don't seem to make sense when compared with this photo. The 1941 Jane's says (also Conway's), and can be seen in their 1935 photo of G-21, as having a twin 25-mm SP gun mount forward and a quad 13-mm gun mount aft. Both guns were French made. It is clear from the photo I found that these guns were replaced with two single 20-mm guns. Most of the online "references" simply added these two 20-mm guns to the original armament, giving an impressive array of guns on these small vessels. But totally wrong. The 1953-54 Jane's issue I have, states that the surviving units as having only the two 20-mm guns.
Apparently they were used as coastal escorts for oil tankers from Mexico to the USA.
These vessels being so small tricked me into thinking they wouldn't be covered in Jane's or other sources.
There is a microfilm collection at NARA of all(?) 19-N photos that were in the BuShips Collection of photos in about 1946, called 19-Z. These images were photocopies taken by a camera of prints. The images are in numerical order, but not necessarily in chronological order. They were numbered in the order "received". The microfilm images are generally pretty poor, but they serve as a resource in locating photos where the prints are missing in the print collections of the 19-LCM (BuShips) photos.
Before NARA was closed for the Damn Virus, I was going through the microfilm rolls these images are on, using an OLD microfilm reader at NARA. Most photos are of USN warships that can be readily identified. Many have labels on them that ID the date and ship. However, I came across some photos that were of ships I didn't recognize and that could be USCG or even foreign ships being worked on in the USN yards. The following three images is of one of these ships. I took cell phone images of the microfilm reader screen (better reproduction job than the print function on this old machine).
Anyone recognize the ship? There are no visible names or hull numbers. Even which yard the photos were taken isn't clear to me. The USN overhauled Mexican, Cuban, and several South American warships during WWII. This ship could be one of those. It doesn't appear to be a converted "yacht", but may have been a private ship acquired as a patrol craft. The armament consists of two 20-mm guns and a few depth charges.