This it to distinguish it from the signet-1.3-n, pictured below.

#m=image%2Fjpeg&dim=1920x1440&alt=A+blue+USB+device+with+a+matching+blue+button.+This+one+has+bolts+visible+in+the+corners+of+the+case+and+a+loop+has+been+attached+to+hold+it+to+a+keychain.&blurhash=%23fMZ1yax%5EbR.a%23WDR-ocjssmaeR%2BWEWXoJs%2Ca%7DWX%7E2Rn9%24t2Rko0ayWXoej%3Dj%5Dj%5Daxocj%5BWEazaeR.jsNHbIt5WEs.oJa%23NKayoej%40oJR-R%2Bj%3Fs.R-j%5Bs%2CWCayoeWXazWC&x=a06f24a4b48bb708de90f89feb01bef4da51deb655935e6a846573660fb9393d

#m=image%2Fjpeg&dim=1920x1440&alt=The+bottom+of+that+same+USB+device.+From+this+angle%2C+we+can+see+the+nuts+that+are+flush+mounted+with+the+bottom.&blurhash=%23bLD3aE9%3D%3B-iofs%2BjEa%7Dofs9WEWroIWBR.ofs%2Cax%7E1xBIuR.R-R-R-WDWCt3obWBflt6jZR*bHkCjYofR.NHWBt5oeoIoJWFR%2BjsbIa%7Es%2BWBR-oKWrbHWCj%3Fs-WCoea%23WD&x=0fe7d9575e0b2bccc48aae6107c5e9666dd13b87fb7be418492535a2161ee43a

This is the older model case which had bolts that were visible from the top and required four M2.5 nuts to hold it together.

I thought the nuts would be needed to avoid the screws from stripping out in the plastic case, but it turns out even the test signet-1.3 devices that I've taken apart and put together numerous times have held up just fine.