The STM8S903K3T6C is an upgraded "application specific" STM8S low-density device with 7 ADC channels, an internal bandgap reference, re-mappable UART pins, and the cascaded timer features of the medium/high-density devices. All other features are identical to the STM8S103K3T6C (pin-out, memory 8K Flash, 640 byte EEPROM).

For the same unfathomable reason that now made the "Value Line" STM8S003F3P6 more expensive than the higher rated "Access Line" STM8S103F3P6 the upgraded low density chip is available for as little as $0.32/piece (I bought my supply at an even lower price), and low-cost break-out boards are suddenly appearing on AliExpress, TaoBao and the like.

I updated the notes on the STM8S001J3 in the Wiki because, when comparing the datasheet, I noticed that this chip quite obviously is an STM8S903 and not an STM8S003: some of the additional features are listed, even if the docs were mostly copied from the STM8S003. I have no idea what this should be good for since a subset of STM8S903 features has been documented, and replacing the chip with generic low-density won't work. Using the bandgap reference works (at 3.3V supply 7 ADC! ADC@ . results in approx. 400), the specified 16bit timer Timer2 is indeed a Timer5 and the 8bit timer Timer4 is a Timer6 (the additional configuration bits are present). For hobby projects it should be safe to use both feature groups. For product use I would talk with the ST sales team first.