“Helm” is interchangeable with “helmet”, but listed by OED as the archaic and poetic form.

• 06.067 and helmets,
• 12.014 could dimly be seen coats of mail, helms and axes, swords and spears hanging;
• 13.037 A light helm of figured leather,
• 13.042 and their bright helms with their tattered hoods,
• 17.066 a stone hurtling from above smote heavily on his helm,
• 18.011 But I have a helm

“helm, n.1.” OED Online. Oxford University Press, June 2015. Web. 25 July 2015.