All labor performed or materials or services furnished by any person entitled to a lien under this chapter upon the same leasehold for oil and gas purposes or the same pipeline must for the purposes of this chapter be considered as having been performed or furnished under a single contract regardless of whether or not the same was performed or furnished at different times or on separate orders, provided that no more than six months may have elapsed between the date of performance of such labor or the date of furnishing such material or services and the date on which labor is next performed or materials or services are next furnished.