Does anyone know which Lee casting mold is for the 7.7 Jap rifle round?

None of them that I know of. What diameter is the throat?

Have you looked at the www.accuratemolds.com catalog?

Accurate doesn't make anything smaller than .30 caliber, although I sorely wish he did. I haven't checked Midsouth, but sometimes they have special runs of custom, non-cataloged Lee moulds for the military rifles.

Gear

You're kind of stuck because of undersized mold. The LEE for the 303 British is suppose to be fat, but it is not. In fact it's not much larger the 30 caliber mold. Now I have an older Lyman 314299 and it does cast fat enough to be useable in 7.7 Jap. My Jap groove slugs at .311. It's tighter then the average 7.7, but you should fit the throat rather then the groove. I would like to tell after fitting the throat that if a bullet isn't fat enough to fill the groove it's not going to shoot well.