The good news is ... there is. The five "line-type" distress signals are an international standard, properly taught to the CAP ground crews.

Heck, even the geocaching coin tag I got in the mail this week teaches that "V" means you need assistance, and "X" means you need medical assistance![1]

The truly bad news - the Civil Air Patrol aren't teaching the people we are signaling for help ( the CAP mission scanners ) what "V" means, and they aren't telling them that when we display "X" we need medical help!

I did more digging, and it looks very bad. All three formats of the Civil Air Patrol Mission Scanner training omit "V" and say "X" means "unable to proceed" rather than the correct "Need Medical Assistance":

(a) The Mission Scanner Task Guide (Dec 2014)[2]
(b) The Mission Scanner "text" (June 2017, no less!) [3]
(c) The Mission Scanner training PowerPoint slides[4]

We'll see if my note to the CAP Academy bears any fruit ...
If so, kudos to Montanero for putting us onto this.

While we're at it - the above are the "line-style" distress signals. For the postural signals: the ground crew is taught all 10 postural signals taught to the Mission Scanner is, but the ground crew is taught an 11th postural signal to communicate to the Mission Scanner (make message drop), that isn't taught to the Mission Scanner, so they may not succeed with that one, either.

[1] I ordered the geocaching coin itself because it was a signal mirror geocaching coin (of course!). The tag is
tiny, and it abbreviated the descriptions of all 3 distress signals on it. It gives "V" as "NEED HELP" vs. "Need Assistance", and "X" as "NEED MEDICAL" vs. "Need Medical Assistance", but that's a big step up from the Mission Scanner training.

[2] The Mission Scanner Task Guide is at:
http://nesa.cap.gov/mas-curriculum-2
under: Mission Aircrew School Curriculum :
Mission Scanner Task Guide - Dec 14

the .pdf download link is:
http://nesa.cap.gov/s/Mission-Scanner-Task-Guides-Dec14.pdf

This topic is under Task MS O-2021 - the pertinent figure is the bottom half of Page 29.

[3] The relevant training text package is on:
http://nesa.cap.gov/mas-curriculum-2
under Mission Aircrew Reference Material & Slides
Basic School - Mission Scanner/Airborne Photographer
Volume 1 - Mission Scanner Text
the .pdf download link is:
http://nesa.cap.gov/s/MART-VOL-I-Scanner-Ref-Text-Rev-June-2017.pdf

The document title, and location of the distress signal table:

CIVIL AIR PATROL U.S. Air Force Auxiliary
Mission Aircrew Reference Text
Volume I Mission Scanner
Revision June 2017
Section 4.2.3 Emergency distress signals, page 55.

[4] The relevant training slide package is on:
http://nesa.cap.gov/mas-curriculum-2
under Mission Aircrew Reference Material & Slides
Basic School - Mission Scanner/Airborne Photographer
Mission Scanner Slide Presentations
Part One

the PowerPoint download link is:
http://nesa.cap.gov/s/CAP-SCN-Rev-June-13-Part-1.pptx
The slide with the missing "V" and mis-defined "X" is slide