Heres the California Vehicle Code for Safety Belts.
Mandatory Seat Belt Law
(Look at section (d)(1) )
27315. (a) The Legislature finds that a mandatory seatbelt law will contribute to reducing highway deaths and injuries by encouraging greater usage of existing manual seatbelts, that automatic crash protection systems which require no action by vehicle occupants offer the best hope of reducing deaths and injuries, and that encouraging the use of manual safety belts is only a partial remedy for addressing this major cause of death and injury. The Legislature declares that the enactment of this section is intended to be compatible with support for federal safety standards requiring automatic crash protection systems and should not be used in any manner to rescind federal requirements for installation of automatic restraints in new cars.
(b) This section shall be known and may be cited as the Motor Vehicle Safety Act.
(c) (1) As used in this section, "motor vehicle" means any passenger vehicle or any motortruck or truck tractor, but does not include a motorcycle.
(2) Until May 1, 2000, for purposes of this section, a "motor vehicle" also means any farm labor vehicle that was first issued an inspection certificate under Section 31401 on or after October 1, 1999.
(3) On and after May 1, 2000, for purposes of this section, a "motor vehicle" also means any farm labor vehicle, regardless of date of certification under Section 31401.
(d) (1) No person shall operate a motor vehicle on a highway unless that person and all passengers 16 years of age or over are properly restrained by a safety belt. This paragraph does not apply to the operator of a taxicab, as defined in Section 27908, when the taxicab is driven on a city street and is engaged in the transportation of a fare-paying passenger. The safety belt requirement established by this paragraph is the minimum safety standard applicable to employees being transported in a motor vehicle. This paragraph does not preempt any more stringent or restrictive standards imposed by the Labor Code or any other state or federal regulation regarding the transportation of employees in a motor vehicle.
Heres the Shoulder Belt Section:
Shoulder Harnesses and Lapbelts
1663. (a) The department shall, in the synopsis or summary of laws regulating the operation of vehicles and the use of the highways published under subdivision (b) of Section 1656, provide a warning which states that, in certain accidents, the lack of a shoulder harness may cause, or aggravate, serious and fatal injuries, especially to the head, spinal column, and abdominal organs.
(b) Nothing in this section limits or impairs the rights or remedies that are otherwise available to any person under existing law.
Used Passenger Vehicles
27314. (a) No dealer shall sell or offer for sale any used passenger vehicle that was manufactured on or after January 1, 1962, other than a motorcycle, unless it is equipped with at least two seatbelts which are installed for the use of persons in the front seat of the vehicle.
(b) No dealer shall sell or offer for sale any used passenger vehicle manufactured on or after January 1, 1968, other than a motorcycle, unless it is equipped with seatbelts for each seating position.
(c) Seatbelts required in subdivisions (a) and (b) shall comply with regulations established by the department.
(d) The requirements of this section shall not apply to sales to dealers, automobile dismantlers, or junk dealers.
Hope this helps. I believe the shoulder belt was a option, since my CS GT did not have them, but my buddies C code did. His CS is dated in March and mineis dated in April. I belive the code is stating that the shoulder belt is optional to wear and thats why theres a mandatory law for the warning labels. Also I believe the Law Enforcement cannot be in conflict with what the Vehicle Code states. As long as you were wearing your lap belt.
Question: do you have a reasonable arguement. (Yes)
As long as you were wearing your lap belt at the time of pull over, then I would say you were legally wearing your seat belt.
As I remember my first car was a 1968 Cougar and there were also no shoulder belts installed.
The other question arises? Was there another reason why the officer pulled you over, because if it is because of the wearing of the seat belt, how could he tell if it was just your lap belt, the officer needs probable cause and seeing your shoulder belt would not be enough probable cause (according to the vehicle code) If there was another violation then he would has probable cause, but not for your lap belt.
Hope that helps. Let us know what the outcome is, Im curiuos!