In simple terms, the shorty is meant to fully thread in to a female NPT fitting far enough that the seal is made via the nested oring. The "submerge" version is extremely similar but the threads are about 3/4" longer so that the unit can be installed as a bulkhead with locknut and washer outside and still leave plenty of thread for another fitting like a valve.
Yes, in theory the shorty can be used as a bulkhead but the threads are intentionally cut small. When you thread the valve on outside the kettle, you'd need to use plenty of teflon tape to be sure the threads seal.