Your transcript is correct, but the author of the postcard wasn't a German native speaker. It must have been someone who just was beginning to learn German. The sentence is full of errors.
So, the complete sentence, corrected (for a male author) and then translated, is:
Ich bin kein Deutscher, sondern Lette aus Lettland, das sich jetzt ebenfalls jenseits der großen Grenze oben an der Ostsee befindet.
I'm not German, but Latvian from Latvia, which now also is located beyond the big border up there at the Baltic Sea.
I think, what the author called the big border was the Iron Curtain, that from 1945 to 1991 separated Europe into a western and an eastern half, where the eastern half was all countries that belonged to Warsaw Pact, among them Latvia, which even was a part of soviet union in this epoch.
Oben (up there) is an often used synonym for nördlich (north). This is because, when you attach a map to a wall, then the northern parts are on the upper part of the map.
Addendum (Reaction to comments)
1. meaning of »ihrerseits«
The word »ihrerseits« has absolutely no local connotation, and therefore also no geographic connotation. It doesn't mean form/at/on her location. It means from/on her part.
Examples for correct usage (Exampels taken from Wiktionary):
Paul hat mir andere Informationen übermittelt als Lisa. Es handelt sich wohl um ein Missverständnis ihrerseits.
Paul has given me other information than Lisa. It is probably a misunderstanding on her part.
Die anderen haben ihrerseits genau den gleichen Fehler gemacht.
The others made exactly the same mistake on their part.
There is also a masculine counterpart for singular usage, which also is used for neuter gender:
- singular feminine:
Lisa hat mir andere Informationen übermittelt als Paul. Es handelt sich wohl um ein Missverständnis seinerseits.
Lisa has given me other information than Paul. It is probably a misunderstanding on his part.
- singular neuter:
Lisa hat mir andere Informationen übermittelt als das Ministerium. Es handelt sich wohl um ein Missverständnis seinerseits.
Lisa has given me other information than the ministry. It is probably a misunderstanding on its part.
The words »jenseits« and »diesseits« do mean locations:
- diesseits = on this side
- jenseits = on the other side
If the author und the receiver of the text both lived in a Warsaw pact state, or even in the soviet union, then of course diesseits would be the word that the author wanted to write:
Ich bin kein Deutscher, sondern Lette aus Lettland, das sich jetzt ebenfalls diesseits der großen Grenze oben an der Ostsee befindet.
I'm not German, but Latvian from Latvia, which now also is located on this side of the big border up there at the Baltic Sea.
If sender and receiver lived on different sides of the border, then both words are possible, this part of the sentence can be interpreted from both sides of view.