I think דר stands for a family name.
I found: תיק יוצר 2064/61שי - note the שי is at the end of the file numbers and seems to pretain to a family name.

The only other guess I would make (but am really not happy with it) is that דר is short for דרגה.
Check out the link and see if it helps. Have you tried to look for the file number on the web?


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Note added at 3 days6 hrs (2008-12-17 21:19:30 GMT)
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I also found the following which may be helpful:
http://www1.landregistry.gov.uk/assets/library/documents/k9....
Look into "key number" in the explanatory notes (2nd page).

Note how in the HEB example in the previous link, next to תיק יוצר it says "שטר יוצר". I seem to see a correlation between this and the explanation of "key number".

Please bear in mind that I'm not sure about this, and your guess is as good as mine.

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Note added at 3 days17 hrs (2008-12-18 08:39:44 GMT)
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Hi, I asked a lawyer friend who said the following:
תיק יוצר זה מספר התיק או השטר שיצר את הרישום או הביא לרישום בטאבו (לשכת רישום המקרקעין).