BMC part number 51K1382. Hex Head Screw 5/16-24-UNF x 1 with 5/8" long thread, 3/8" unthreaded.

I have seen stretched bolts in this application, because people sometimes use the wrong screws. In the annals of MG, this particular fastener has always been a special part number. because it is high strength, and the un-threaded length is special. This fastener is 1" long with 5/8" thread and 3/8" un-threaded.

By definition a "screw" has full threads, regardless of length (maximum of two imperfect threads under the head). A "bolt" can have an un-threaded portion under the head, depending on length of the bolt. For a 5/16" bolt, the minimum thread length would be 1-1/8". Therefore there is no such thing as a 5/16" bolt less than 1-1/4" long. Shorter ones would be screws (by definition). This fastener is a non-standard "bolt", or non-standard "screw" depending on your point of view). The SPL calls it a screw.

If you want to use a standard fastener (which would have full thread), use Grade-8.