Thank you. I can confirm the ID and add to my collection.

If you're going to collect EGAs, the cost of the book is less than one mistaken purchase can be.   5, 8, and 13 are the only post-WWII...most of these are pre-WWII. I wouldn't call any rare, per se, but some are certainly uncommon/more difficult to find   There's a lot here to unpack in one post. Recommend you either break these down and ask one or two at a time and/or use the search function in this forum, every pattern you've posted has been discussed here in the past.

Seen this, if this is correct then it seems they only  made buttons in WWII.   Jaffe was a manufacturer or distributor of U.S. military buttons, often found on vintage or World War II-era Army brass uniform buttons, sometimes in conjunction with manufacturers like Waterbury.   "Jaffe" appears in backmarks (the text on the back of the button), marking them as part of the production of military insignia, often in NYC.

What's interesting is that FOLD3 does not give any relevant US Navy information for Albert Charles Hauck. AI search results are often muddled and nonsensical.   Assuming that there may be some uncertainty as to his year of birth, I found this on ancestrydotcom for August C Hauck born 11 March 1908 (versus 11 March 1906 given in the Naval Registers):   Death index:   Social Security Record:   1950 Census that shows him living in Florida:        

Thank you for giving me the information and where you got it from.  I wasn't doubting you at all but even after you've given me the information and where you got it from I'm still not finding what you're finding.  I'd be interested to know if anyone else on this forum can find what Laurent is seeing?  On Ancestry.com I'm not finding an Albert C. Hauck born in 1913 and who died in 1993 no matter where they were born.  I looked in the Internet Archive at more than one of the The Register of Commissioned and Warrant Officers publications and did a search for just Hauck but could only find one August C. Hauck who was born in 1906, so I don't know.  I must be having an off day or something.      

These are steelbacks with the same backmark.   

Thank you for the comment. I would love to buy a copy of USMC Eagle, Globe, and Anchor Emblem Pictorial Guide", but they are pretty expensive. I'm hoping I'll be able to get a deal on a copy in the future.

A new addition, an experimental 1906 Rock Island Arsenal .38 holster, in Scott Meadows' holster books he writes that 180 were produced and that they were tested by the 13th cavalry, he also shows are a couple of great period photos of the holsters. Originally but now missing there would have been a leather tab at the bottom with  a tie down thong. It ticks two boxes for myself being a keen cavalry accoutrement collector along with a preference to 13th cavalry related items.

You have a model 1840 heavy cavalry saber which looks to be in nice shape. It is likely a European import.  Is there a marker mark on the blade or letters on the pommel cap (end)? A picture of the scabbard l would help  verify country of origin.