Subject to agreements made under chapter 12 of this title, the seniority of an employee of the Postal Service occupying a position whose regular duty involves the collection and delivery of mail on a rural route shall be preserved. Seniority for such employee shall commence on the first day of his service in such a position, or, in the event such an employee transfers to another such position, on the day he enters duty in the other position. Upon initial assignment, such an employee shall be assigned to the least desirable route and shall attain assignment to more desirable routes by seniority. Promotions and assignments for such an employee in such position shall be based on seniority and ability. If ability be sufficient, seniority shall govern.