AJ, the D is not a model indicator, it is a batch series indicator, Saito makes the engines in runs or batchs. The first run has no letter on the right lug, the next has an A etc. If an has improvements such as a 125 became the 125a when Saito added two or three cooling and made the cooling fins deeper.
125 left, 125a on right.
These are from first run and are high compression Saito 150's, they have no initial on the right lug. The one on the right is the first 150 sold by Horizon.