An infantry regiment typically consisted of three battalions - numbered 1st through 3d - each of which consisted of a headquarters and four line companies (three rifle companies and one weapons company). Line companies were designated A through M, excluding the letter J, so that 1st Battalion consisted of Companies A through D, 2d Battalion consisted of Companies E through H and 3d Battalion was made up of Companies I, K, L and M. Companies D, H and M were the weapons companies of their respective battalions.
World War II shoulder patches - even reproductions - are expensive. Likewise, a WWII-issue distinctive insigne for 115th Infantry (there was no shoulder patch). I can check prices with a few collectors.