I have two SV cylinder heads - one has a thimble-like insert or plug in the large water hole located in the middle of the bottom of the head while the other just has a hole with no insert. Could somebody please tell me which is correct (or are both correct) and the purpose the insert/plug is supposed to serve. I would like to re-build the head without the plug but am unsure if it would be wise to do so since a part may be missing. See photo. Thanks.