The Mullard Blackburn EL34 Double O Getter Xf1s are the best a musician can get.  Both have brown base and pin hole on bottom. One tube is larger than the other.  Date codes are  XF1 B1D4 & XF1 B311. I tried taking pictures of the codes but it's very hard see. I found these two gems together in a china cabinet at radio repair shop estate sale.  They must have been special to the owner.  I have not tested these but I haven't found one so far that wasnt good if not great.  There is some writing on one tube that i can't make out but i left it there for next person to see. 

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