The Gamma series had a wide range of cowlings in relation to the model's engine being either a Wright or a Pratt & Whitney.
In the case of an air intake, if the model had one (few did), it was set back, behind the cowling. A clear example of this, would be the Gamma's USAAC derivative, the A-17.
The Gammas also had either an extreme setback to the cockpit or it was moved virtually to the firewall.
Not sure what to think about the "fledgling civil airlines" part, as the Japanese aviation industry was on a par with most other nations in the 20's and 30's.
The Gamma 2E, which also saw service with the Chinese Air Force also saw one each going to the British and Soviet Union for evaluation. Powered by a Wright R-1820-F53.
The single Gamma 5A built, was powered by a Wright R-1820-F52. In the photo, is the aircraft sold to the Imperial Japanese Navy in 1935, registered as BXN1. It was destroyed in an accident during testing.
The Gamma 5B was powered by a Pratt & Whitney R-1535-98 and only one airframe was built. After a brief tour of demonstrations in South America, it was sold to Spain in 1937, where it served with the Spanish Air Force flying coastal patrols.
The only Gamma 5D airframe made, was powered by a Pratt & Whitney R-1340-S3H1 and was initially intended for the Imperial Japanese Army, however the Imperial Japanese Navy took ownership of it in 1936 and gave it the code BXN2. The IJN conducted several tests and then transferred it to Nakajima, who found it useful in their development of the B5N. It was later turned over to the Manchurian Air Lines in the late 30's, who used it for reconnaissance in China and the Soviet Union.
None of the Gammas look close to the Mystery Ship..there are some similarities, which were common across the entire aircraft industry in the 1930's.