body tubes

Hello fellow rocketeers, hope you are doing well. As I kept designing my future rocket, one doubt came into my mind when looking at my BT and MMT. First of all, my BT is 1,968" OD and 1,85" ID; my MMT is 1,574" OD and 1,496" (38mm) ID. The thing is, there is only left 1 3/8 or 1,377" (3,5 mm)...

The other day, I was generously donated some spare 4" body tubes (and one coupler) for building an L1/L2 bird. However, I'm not entirely sure what they're made of, and because they're black, they don't photograph too well. Things I know: They're from Madcow Rocketry. They're 60" long and 4"...