Actually, no....

Lakes operates their 1900s Part 135 now, therefore, an FO on the 1900 at Lakes with 1000 hours of SIC time would not have any credit time for 121.436.

To take it a step further: Why would the 1900 CA at Lakes qualify for 121.436 and not the 1900 PIC hauling boxes?

Answer: The 1900 CA in pax operations is required to hold an ATP certificate to conduct such operations. The 1900 cargo PIC is capable of operating under just a CPL.

The way I see it, the FAA wants time where you were either..:
- In a 121 environment which ensures you have an ATP/R-ATP, and thus the experience required to achieve such certificate.
- In a 135 PIC environment which requires an ATP, and thus the experience required to achieve such certificate.
- In a 91k PIC environment which requires an ATP, and thus the experience required to achieve such certificate.

... and that's the time you can count towards 121.436.