At the Hosoo Pass Postal Exchange ruins, there's a sign board that details the history of the Pass and the Postal Exchange. Below is a translation of what it says. Please read the full post for context.
村指定・史跡細尾峠旧道
指定年月日・平成一三年一二月一八日・平村
Village Designation / Historic Site of Hooso Pass's Former Road
Date of Designation / Dec 18th, 2000
城端から上田と通り、梨谷に出る細尾峠の道は、明治二十年に開通した新しい峠道で、古い絵図には道に記されていない、これまでの朴峠道に変って、新たに開かれた峠道です。
The Hosoo Pass Road, which runs from Johana Town through Ueda and ends in Nashitani, first opened in 1887 (Meiji 20). It replaced the previous Haku Pass Road, which was not marked on old maps.
※峠道が開かれた理由 (Reasons for opening the Road)
大量の製品や原材料が城端町判方商と取引きされるようになり、馬が通行できる道となる。輸送力は増大し、雪前の危険の少ない尾根道は、冬期の通行も比較的容易となった。
Large Quantities of products and raw materials were traded with merchants in Johana Town, and this road made travel accessible to horses. Capacity to transport these goods increased, and this road was less dangerous before winter, and relatively easy to use in winter.
※峠の交通事情 (Transportation at the Pass)
昭和二年に城端、下梨間に自動車道（八幡道路）が開通したが、細尾峠を越える物資や人の往来がまだ盛んであった。冬期間でも峠道が大切な交通路であった。二八年に国鉄バスが開通するや往来が急速に少なくなった。
In 1927 (Showa 2), a paved motorway (called Yahata Road) was opened between Johana Town and Shimonashi Village, but the traffic of goods and people over Hosoo Pass was still active. The pass road was still an important transportation route, even in winter. But, as soon as the JNR (Japanese National Railways) bus service was opened in 1953, traffic on the pass rapidly decreased.
冬の間は自動車は通行できず、一部には庄川沿いに下り、小牧ダムの速絡船の利用もあったが、多くは細尾峠の徒歩交通に頼らねばならなかった。
During the winter, cars weren't allowed to pass through the pass, and although some people went down to Shogawa River and used fast boats from Komaki Dam, people had to rely on transportation by foot to use Hosoo Pass.
※郵送隊の峠越え (Postal Corps crossing the Pass)
一人当り一八キロの郵便物を背負って隊列を組み、細尾峠を越える人々を呼び、城端から平、上平への郵便物の輸送にあたった。明治五年、下梨に平郵便局されて昭和二〇年までの間は、人夫が二人で一組となり、片道二〇キロの山道を通して行われていた。
To transport mail from Johana Town to Taira and Kamitaira. members of the postal corps formed a line and carried 18 kilograms of mail each. They called out to the people who crossed Hosoo Pass. In 1872 (Meiji 5), when the Taira Post Office was established in Shimonashi, mail carriers worked in pairs and traveled 20 kilometers round trip along the mountain roads until 1945 (Showa 20).
※郵便物交換所 (Mail Exchange)
昭和二〇年からは、平、城端局からそれぞれ午前九時に出発し、仲間点の細尾峠で受道をする交換所に変更された。やがてバスの開通によって夏期はバスに託送されたが、一二月から四月の冬期間は人夫による輸送が統いた。高度経済成長期の三〇年後半から小包みが急増、その為臨人夫を加えた。細尾峠を越えた郵送隊は道谷を通り鹿熊峠を越えた。
From 1945 (Showa 20), the post office was changed to an exchange office where mail would depart from Taira Village and Johana Town at 9am and be received at this point in Hosoo Pass. After the bus service was opened, mail was sent by bus in summer, but postal workers still carried the mail by foot in winter from December to April. During the period of high economic growth in the second half of 1955 (Showa 30), the number of small parcels increased sharply, so many temporary workers were hired. After crossing Hosoo Pass, the postal corps passed through the valley road and crossed Kakuma Pass.
※郵送隊の廃止 (Abolishment of the Postal Corps)
郵送隊は上田、見座、相倉の人々で、上田部落では過疎化が進み、輸送人夫の老今化がみられ庄川沿いの道路改修が進んで、昭和四六年春で長らく続いた郵送隊は廃止された。
The Postal Crops was made up of people who lived in Ueda, Miza, and Ainokura, but depopulation occurred in Ueda and the current members were aging. Road repairs along the Shogawa River also progressed, so by 1971 (Showa 46) the postal corps was abolished after its long historic operation.
Note: The post is a stub. In the future, more information could be added to create a full post.