This is the officially shared schematic of the converter board: Raspberry Pi Switchboard SCH_20171218.PDF
Does this match with your findings?
Found that there are some other annoying things, like the I2C pins are misused for something else, had to use
dtoverlay=i2c-gpio,bus=3,i2c_gpio_sda=23,i2c_gpio_scl=24
for assigning new pins. Now I can connect a BME280 to go with it in the outdoor box and report the box conditions.