I figured special ops but now that you mention it I agree that he is MARSOC not Special Forces.
Here is another MARSOC Marine in similar uniform.
IANAMM, but my understanding is that when you drop a mortar round into the tube, you hold the round in both hands and, starting above the muzzle, move your hands down along the outside of the tube in one continuous motion. This allows the round to drop into the tube while ensuring your hands are out of the way of its swift exit. It looks to me like the round had fired a fraction of a second before that picture was snapped, and the soldier simply hadn’t started moving for the next one yet.
Thats just a normal radio. Man portable radios dont take a rucksack to carry anymore. In the background is the antenna for sattelite communications to reach further distances and overcome line-of-sight issues.
That site labels him as “A U.S. special operations forces member.” I still think you are right about MARSOC.
Website of Marine Corps Forces Special Operations Command..
How dare you guys make me move my fingers to a search box to figure out what you are talking about. 
Been learnin’ stuff.
For GI Marines, GPS is not standard gear, at least not “commonly,” going by the penultimate item in the ADVANCED MORTARMAN COURSE GEAR LIST.
Answered my OP on how many per unit here, with pictures, from the Corps.
More on what Uncle Sam thinks about these guys from an administrative point of view:
From [MILITARY OCCUPATIONAL SPECIALTIES MANUAL (SHORT TITLE: MOS MANUAL)](MILITARY OCCUPATIONAL SPECIALTIES MANUAL (SHORT TITLE: MOS MANUAL))
(P. 329 on PDF link):
- MOS 0341, Mortarman (Sgt to Pvt) PMOS
a. Summary. Mortarmen are responsible for the tactical employment of the 60mm light mortar and the 81mm medium mortar. Mortarmen provide indirect fire in support of the rifle and LARS quads/platoons/companies and the infantry and LAR battalions. They are located in the weapons platoons of the rifle and LAR companies and the weapons company of the infantry battalion. Noncommissioned officers are assigned as mortar gunners, forward observers, fire direction plotters, and squad and section leaders.
b. Prerequisites. Must possess a GT score of 80 or higher.
c. Requirements. Complete the Mortarman Course at the School of Infantry, East or West.
d. Duties. For a complete listing of duties and tasks, refer to reference (g), Training and Readiness Manual.
e. Related SOC Classification/SOC Code. Infantry 55-3016.
f. Related Military Skill. Field Artillery Cannoneer, 0811.
Also see
Marine Corps Enlisted Job Descriptions
MOS 0341 – Mortarman
By Rod Powers, About.com Guide, which has same but hyperlinked.
Ain’t Google grand once you get interested in some things?
Plus, that MOS Manual is an amazing and fascinating document. Imagine all the similar ones that are classified.
ETA: The Training and Readiness Manual was not available on-line (how do you say “shucks” in Arabic?).
Just realize that special ops has their own rules.
That looks like a PRC-117 antenna.
Bear_Nenno beat me. He’s wearing a GPS on his wrist, and Merrell brand boots on his feet. It’s not Gucci gear, per se, but damned comfy and necessary if you’re on the move-and-shoot.
Tripler
Mmmm, Merrells ::drool::
The circular polarized antenna on the ground in the background might be from a 117 but the ‘measuring tape’ antenna on his back is more likely from a PRC-148 / MBITR.
The 117 is certainly a manpack form factor for satcom and the AN/PRC-150 is manpack’d for long distance HF (shortwave).
If anyone has a spare '150, please drop me a line
Mmmm, Harris HF manpack… ::drool::
More likely that this is attached to a PRC-152, not a PRC-150. I don’t know anyone who carries such a large radio outside of a vehicle anymore unless their job is specifically radio communications (FISTER or FO/RTO, etc).
Hip fired.
Leo Bloom, the US Army Field Manual FM 3-22.90 is approved for public release. Would you like me to email it to you? You might be able to find a copy online, if not, PM me.
Also, what has made you so interested in mortarmen?
My guess was that he’s holding onto the tube with both hands to steady it so the recoil doesn’t knock it off-target.
Re the shoes I sell shoes on ebay as a hobby business and I have sold many pairs of lightweight hiking style boots to troops overseas. Getting ebay shipping to resolve those funky military addresses is PITA as the shipping apps keeps throwing up when I try to print the label. I wind up hand addressing most of the shipments.
Jesus, you two–put 'em back in your pants.
Military geeks. Possibly the worst combination of all. 
Well, as the shooter says, it’s doable. Like to see a shoot-out in the Old West where on “draw” each reaches for their holster.
Bear_Nenno, thank you for your kind offer. I remember your offers of help from an earlier thread on calling off marching cadences, where your generosity was equally quick.
What made me so interested in mortarman? Saw a picture of something I didn’t understand and thought was interesting, got more interested because of the responses in this thread, and it was off to the races. I like learnin’ stuff, as I announced.
I’m sure an approach to research shared by many on this board.
I’ve never heard the word before (but the meaning is clear in this case), but “manpack” alone is one hell of a word to drool over, if you’re into that sort of thing.
The only thing that could make it better would be more words that have “k” sound at the end, for even more manliness (Chuck Beefcake,* eg.).
“Check out my manpack, why don’t you? Some form factor, huh?” (Plus the “f” with the “k” is the cherry on the cake.)
*See The Nine Manliest Names in the World.
BTW, the bit about the “k” sound holds true in expressive words on sex, curses, and bad words in general in many languages
This guy is almost certainly not a mortarman though right? Don’t the special forces guys just get special training on the weapons they use?