There are two variations in which an E/63 magazine would support a U-Block. The first is a 1937, 2nd Variation and the other is a 1939 Code 42. There is a slight difference in the two, which is the size of the acceptance mark. The 1937 stamping is smaller that the 1939. The photo tells me this is the Larger of the two and rightfully belongs to the 1939.
There were only 12 receivers that would be available to provide both the 1939 chamber date and the 5084 reciever number, so the chances of it being a different receiver are remote.
The best bet would be a changed toggle assembly or toggle link.
Now, I really hate to muddy the water, but . . .
Jan Still reports what he calls "out-of-sequence" Production, Page 91, Third Reich Lugers. Here he shows a 1940 Code S/42, 1940 byf and a 42 Code 42. Now who knows how many of these exist and if they are indeed legitimate. Now that's 3 pistols in 300,000 or more Lugers. I guess if one would belive the existence of a 1949 Code S/42, a U-Block 1939 Code S/42 is possible.
The pistol in question could be either an out-of-sequence or it has a toggle transplant, and which is original finish or reblued.
I guess at this point, you pays your money and takes your chances. IMHO it is worth more than $500, but would hesitate to spend more than $1000.
Nuff Said, at least from me !! <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />