WC55 with a M50 ring mount? It would have severely limited the 37mm M3 gun's arc of fire. If you look at a WC55 and then a ring mount off a WC62/63 - you will immediately see why the mount was not used for the WC55. Also, the M50 ring mount came out over 2 years after the WC55 - so not a factory setup. Also, the M50 ring mount (uses the M49 ring) was designed specificly for the 1.5 ton WC. Finally, the M50 was authorized for the WC62/63 in 1944 per Organization Control Manual (OCM) 24052 issued April 22, 1944. Prior to that, the WC62/63 was only authorized for the M24A2 mount just like the WC 51/52/55, which mounted to the front of the cargo bed, see OCM 22263 issued December 2, 1943. Actually - I found a Life photo from Italy 1944 of a WC55 showing what I meant about lack of room for the M24a bed mg mount or the M50 support legs for a ring mount with the 37mm gun in place.
http://images.google.com/hosted/life/f?q=armed+forces+source:life&prev=/images%3Fq%3Darmed%2Bforces%2Bsource:life%26start%3D72%26ndsp%3D18%26hl%3Den%26sa%3DN&imgurl=96a3df6ee60b38e5
And yes, there is that oddball photo of the 1942 desert (unsure if it was Nevada or CA) training with a WC55 with a 50 cal mounted on a post on the front bumper with the gunner hanging off it. The mg post seems to be a unique mount - as it does not appear to be referenced in the huge WW2 ordinance mount manual covering all authorized mg mounts used in WW2 (the version through 1943 is TM 9-224 - Portrayal Press has it for purchase. [At Wayne's request I deleted the link to the Steelsoldiers site for it.]). Maybe the post mount was a custom fab for a publicity photo?
The WC55 was found to be abdismal failure against German armor in North Africa, so almost all converted to WC52 cargo carriers. However, some WC55's made it to the ETO - the web has a great shot of 10 or so lined up on a French road loaded to the gills with supplies shortly after D-Day.