Unless expressly prohibited, a motor common carrier of property holding separate authorities which have common service points may join, or “tack,” those authorities at the common point, or “gateway,” for the purpose of performing through service as follows:
(c) irregular–route authorities may be tacked
with one another if the authorities were granted pursuant to applications
filed on or before November, 23, 1973, and the distance between the points
at which service is provided, when measured through the gateway point, is
300 miles or less; and