To make things complicated, be aware of old, ancient and new measurements.
Scaffold tube in 26,9mm will have wall thickness of 2,65 mm meaning inside diameter will be 26,9 - 2,65 - 2,65 = 21,6 mm which IS NOT 1 inch or 1"
1 inch = 25,4 mm
If indeed you have 26,9 out- and 25,4 inside then wall thickness = (26,9-25,4) / 2 = 0,75 mm … Totally unusable.
My guess is (since you mention plumbing) that you have 1" galvanised tube which would not necesseraly be an inch but might also be reffered to as “thumb”.
These pipes are used in , for instance, sprinkler systems as galvanised steel pipe is NOT used for drinkingwater (there you would use copper or plastic (tyleen) tubes)
Dependig where you live , 1 thumb (written 1") can be anywhere between 24 and 27 mm…
Swedish wiki for instance : tum – Thumb (inch), 1/12 fot, 2.474 cm. After 1863 1/10 fot, 2.96 cm, not much accepted by professional users in mechanics and carpentry who later switched to English inch (2.54 cm, abandoned only late 20th century) and metric system.
Anyway , if you take an average 1 thumb ( 1") tube it would typically have an outside diameter of 33,7 mm …