Hi - I am new to the forum. Over the last 25 years, I have been buying and selling various mechanical parts for North American cars and trucks. Somewhere along the line, I acquired a pair of bare heads that were sold as and appear to be new old stock. I was told that they were for a 350 diesel. Casting number found under cover and at bottom read 3948 209, or perhaps they should be read as 3948209. GM and D3A are also found on the castings. What are these? Any value? Any help would be very much appreciated! Thanks