You may have to call Apple on this one. EveryMac doesn't tell and OWC is not listing one:
I really need to know the exact MacBook Pro you have. But it looks to me like the 3.1s have on board solder in memory.
Standard Storage: 128, 256, 512 GB SSD Std. Storage Speed: 643.6 MB/s/1.3 GB/s*
Details: Storage varies depending on the stock "base" configuration before the custom processor is selected. Originally shipped standard with 128 GB (MF839LL/A), 256 GB (MF840LL/A) or 512 GB (MF841LL/A) of flash storage. The MF841LL/A option also could be configured with 1 TB of flash storage for an additional US$500. The price of this 1 TB upgrade effectively was cut to US$400 on October 27, 2016.
iMac 5K uses
Standard Storage: 2 TB (Fusion) Std. Storage Speed: N/A
Details: This model is equipped by default with a 2 TB "Fusion" Drive, which combines a 128 GB SSD and a 2 TB hard drive. It has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD.
RAM Type: PC3-14900 DDR3 Min. RAM Speed: 1867 MHz
Details: Supports 1867 MHz PC3-14900 DDR3 SDRAM SO-DIMMs (204-pin).