In a direct communication, the source confirmed:

Treasury certificate series are discontinued on 30 December of 2016. 

In response, we have marked the series as "discontinued" in its metadata and the catalog. Figures are percent per annum (%), not seasonally adjusted (NSA). The series runs from 19 February 1991 to 16 December 2016. 

The series resides in the historical catalog (Belgium » Financial » Interest rates » Sovereign yields) and is:

• IR%MMTC1WUD.IBEL = [DISCONTINUED] Money Market Rates: Reference Rates of Treasury Bills with a maturity of 1 week, (%, NSA)

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