[jmacusa]

I didn't say “regiment’’. I was under the impression Marines often just say,as an example , ‘5 Marines’’, it being understood, certainly between Marines anyway,that it means ‘’Division’’.

[oh8eleven]

I didn't say “regiment’’. I was under the impression Marines often just say,as an example , ‘5 Marines’’
You did when you said, "My late-father-in-law was 4th. Marines." What you're saying is that he was with the 4th Marine Regiment.
You also said your brother was with 5th Marines, 3rd. Battalion. That unit would be known as 3/5, or from a regimental level - 5th Marines.