The T21s at HCGIS Detling did not have facility to fit "the bullet" into the nose in 1955 nor did the T21s at 662 GS in 1957.
The T21 we had at Benson (RAFGSA Club) did not have the nose ballast mod in 1963 when I joined and students did their first solo in a Grunau Baby (with sometimes exciting results).
My best guess is that the nose ballast mod came along around 1965.