The OS F plugs are for 4 stroke motors but work great for 2 stroke motors that are mounted inverted. The F plug has a longer element that generates more heat which is needed for 4 strokes, but for inverted 2 stroke motors, the extra heat helps idling and transition since there is natually more fuel / oil gathering at the head.

Regarding price, OS and YS are made in Japan (higher cost) while ST are now built in China (lower cost). ST never went out of business. They were originally made in Italy and were around for a long time, but they moved operations to China a couple years ago. Probably to reduce cost and price to improve competitiveness. During the transition, it was hard to find some of the ST motor sizes especially the 51, since it was so popular before.

Give the ST 51 a try. I think Tower Hobbies has some discounts if ordered before the end of this year.