Hi Florian,
I've been searching on a similar theme recently, and wondered if this would help.
『刀剣と歴史 (536号)』(日本刀剣保存会刊)
浦茂(元中佐)「日本刀保存についてマッカーサー司令部との交渉経過(当時の日誌から)」
A memoir by Shigeru Ura (1909-2001, ex-military), "The Process of Negotiation on the Preservation on Japanese Swords with MacArthur Headquarter (With Reference to My Journals from 1945)"
https://asahitoken.j...sm-katanagari-C.html
The Journal 『刀剣と歴史』seems to be published by 日本刀剣保存会 (NPO), but they don't have any online archive.
(Their website is still working on the English version)
http://hozonkai.jp/3book.html
Also, SCAPIN orders like NO.50, 181 and 574 can be found here. (National Diet Library's website)
(NO.2099 includes the list of SCAPIN orders related to instructions on surrendering arms)
https://rnavi.ndl.go...pation/jp/SCA-1.html
Wish you best luck!
Eugene