I thought 135 pax ops required an SIC regardless of type if there's no autopilot. I'll have to look again though, I try not to clog too much of my brain with that stuff.
You are correct. It seems like this question is asked about once a day on various forums, and it seems that it is usually answered incorrectly or at least incompletely. There is no OpSpec requiring two pilots for 135 IFR passenger carrying ops. The 135.101 already requires it. The OpSpec concerning autopilot use is there to allow you to deviate from that requirement if you choose to do so.
§ 135.101 Second in command required under IFR.
Except as provided in §135.105, no person may operate an aircraft carrying passengers under IFR unless there is a second in command in the aircraft.
[Doc. No. 28743, 62 FR 42374, Aug. 6, 1997]
§ 135.105 Exception to second in command requirement: Approval for use of autopilot system.
(a) Except as provided in §§135.99 and 135.111, unless two pilots are required by this chapter for operations under VFR, a person may operate an aircraft without a second in command, if it is equipped with an operative approved autopilot system and the use of that system is authorized by appropriate operations specifications. No certificate holder may use any person, nor may any person serve, as a pilot in command under this section of an aircraft operated in a commuter operation, as defined in part 119 of this chapter unless that person has at least 100 hours pilot in command flight time in the make and model of aircraft to be flown and has met all other applicable requirements of this part.
(b) The certificate holder may apply for an amendment of its operations specifications to authorize the use of an autopilot system in place of a second in command.
(c) The Administrator issues an amendment to the operations specifications authorizing the use of an autopilot system, in place of a second in command, if—
(1) The autopilot is capable of operating the aircraft controls to maintain flight and maneuver it about the three axes; and
(2) The certificate holder shows, to the satisfaction of the Administrator, that operations using the autopilot system can be conducted safely and in compliance with this part.
The amendment contains any conditions or limitations on the use of the autopilot system that the Administrator determines are needed in the interest of safety.
[Doc. No. 16097, 43 FR 46783, Oct. 10, 1978, as amended by Amdt. 135–3, 45 FR 7542, Feb. 4, 1980; Amdt. 135–58, 60 FR 65939, Dec. 20, 1995]