I can understand the Ichiguchi call, based on the shape and workmanship of "Masanobu" (市口明珍正信), a recorded Ichiguchi Myochin smith from Edo. See picture below:
Although totally diffrent from Fujiwara Masatsugu (藤原正次), also Ichiguchi. But that leads too far.....
My Owari search ended unfortunataly in a blind alley....C’est la vie
However, the interesting point is, that the kabuto I postet above is signed "Masanori saku" (正則作) and sports almost the same characteristics as the mentioned "Masanobu" example and the kabuto in question. "Masanori" is not listet as an Ichiguchi smith. But because he uses the same character for "Masa" (正) in his mei, like "Masanobu", we can probably assume, that he was also in the lineage of the "horsebit guys".
All in all, despite the odd mei "Terunaga" (輝長?), I have to admit John, that the most indications pointing towards "Ichiguchi Myochin" (市口明珍) at the moment
Nick, can you make a close up of the mei? Hard to read!