As given in Burke, the fifth Royal Tribe of Wales is as follows:
Jestyn ap Gwrgant, Tributary Prince of Glamorgan, Founder of the V. Royal Tribe
Gu. three chevronels ar.
This would be a red shield with three pyramid shaped set of parallel lines starting from the bottom right and left of the shield, meeting in the center at equal distance.
The Chevron is thought to have been adopted from the bow of a war saddle, which rose high in front. [Burke, p. xxxi]
The reference is : Burke, B.B., The General Armory of England, Scotland, Ireland and Wales; A Registry of Armorial Bearings From The Earliest To The Present Time. Harrison, 59, Pall Mall, 1884. p. lxii.