I would just shoot for 229.52 which supersedes 229.51. Check the approval lists, any 229.52 oil on the approved list should be fine. Basically 229.5 is best for gas, 229.52 is best for diesel even though you could use 229.51 or 229.52 in gas engines.No, two different applications.
MB229.5 is current, but so called High-SAPS oil, intended for vehicles without SCR/DPF or GPF (like yours).
MB229.51 is Low-SAPS oil approval or as some would say "diesel" oil. It is for vehicles, both diesel and gas that have aftertreatment equipment (DPF/GPF).
Bcs. your vehicle is diesel, regardless that you do not have DPF, I would still go MB229.51/MB229.52.