这个问题的解释最好是上面两位老师观点的综合，即第一，这是存在句型应该是没问题的，第二，这也是倒装。第三，楼主认为是a preserved letter不合理，正常的是 a letter preserved,至于怎么分析preserved,可能会有一些看法分歧，但我倾向于看作被动语态或补足成分，又或者是小句，但不是定语后置。（虽然有些情况可能是后置定语。）下面我分别举例说明。

第一，存在句的形式很复杂，我只介绍不常见的相关例子 ，即及物动词出现在存在句，正常情况下是不被允许的，但是被动态的及物动词是常见的。比如 In recent years there has been produced more food than the country needs. （徐广联）） There‘s a new grammar been written.（夸克）当然，有人认为不是被动态，而是名词后的分词短语出现前置情况，这个自己判断吧，确实情况各有不同。以下例句类似楼主的例子There was presented (to the winner) by the mayor a gold medal. (Quirk et al. 1985 1409) --There was a gold medal presented to the winner by the mayor.(Quirk).There has been a whole box stolen. There was presented to me a papaya. There were shown to us, by the diggers, several interesting specimens.(Bolinger)。当然我们最常见的情况是分词后置There has been a wooden bridge built over the river. There's been a terrible mistake made. Has there been anyone killed? In each case there has been a serious mistake made. There has been a massive welcome organised. There has been a whole box stolen.=A whole box has been stolen. 附带：注意，有些情况下，被非宾格限制的及物动词完全可以被接受。(这些特殊情况应该是只做了解，切勿模仿）如：Then，all of a sudden, there reached her ear the sound of angel voices.  / At this point,there hit the embankment a shell from our own lines.( Kuno & Takami 2004 :40)

第二，当there存在句的名词短语太长时，作为后置修饰语的分词也可以前移至名词短语之前，使句子显得较为平稳，合乎句子的末端尾重原则。

There were running in the garden a group of children aged from seven to eleven.

There was shown at the exhibition an electronic computer made in Shanghai.

倘若名词短语的修饰语太长，这种分词的前移甚至是强制性的：

From the Norweigan,Dannish and Iceland historians,and from some parts of the old Northern Poetry, there may be found a different idea about the character and domestic manners of the man who made themselves so unpleasant in their visits to the English and neighbouring coast. 

There has just been published as a Parliamentary paper the Convention between this country and the United States respecting the Protection and Propagation of Food Fishes in the waters contiguous to the United States and Canada. 

也有人认为，这种句子系属there存在句的被动式和进行体。

第三，preserved如果被看做是前置定语，意义很奇怪，不合逻辑--在华盛顿的文件中有一封被保存下来的信件（他保存的吗？显然不是）所以，应该是一封信被保存下来。这个preserved 表示的是信件所处的一种状态。这里想附带说明的是，国内语法有说明的多是把分词短语看作后置定语，这个其实很不合理，这个后置短语（分词，介词，形容词等）大多表示的都是逻辑主语所处的状态，而不是对其的描述或限制。当然也有情况是后置定语，这个要看上下文的具体情况。我引些例子：(8) a. There was a book completely burnt to ashes.

b. There was a live pig roasted. (Milsark 1974: 84)

Under the depictive analysis Hartmann argues for, (8a) is predicted to assert that there existed a book in a

completely burnt state. Our intuitions tell us, however, that the sentence actually asserts that there existed

some event in which a book was completely burnt. Similarly, (8b) is predicted to assert the existence of a

live pig in a roasted state. But according to our intuitions, the sentence asserts instead that there was an

event of a live pig getting roasted (and, consequently, the pig ended up in a completely roasted state). As

far as we can see, there is no way for the depictive analysis to overcome such inadequacies.  （这段话我就不翻了）

There were two people absent.  Huddelston2002 认为，形容词扩展成分应该是一个单独的句子成分，而不是动词后的名词词组的一部分。分词也是独立于存在句中的动词后的名词词组之外的成分，

个人观点，仅供参考。

感觉有些像there be结构的变体。常见的是there be中的动词可以替换为exist、live、stand等不及物动词。这里则采用了及物动词的被动形式。这个感觉除了来自不怎么靠谱的语感，主要是基于句子里已经有地点状语，there似乎没有实际内容。

个人不成熟的看法，仅供参考。