How to design a database that easily lets me recreate historical snapshots

From what I can understand, apart from a Customer table, you'll need three tables:

1. Contract — holding the single-valued data about a contract, excluding receipts. It might record the latest receipt number for the contract, but that would be an optimization, storing derivable data. Primary Key: Contract Number (aka Lot Number).
2. Contract Items — holding the 1-6 items for the contract. Primary Key: Lot Number, Lot Sequence Number. Lot Number is a Foreign Key reference to Contract.
3. Receipts — holding information about receipts. Primary Key: Receipt Number. Foreign Key: Contract Number reference to Contract again.

A given receipt is associated with one contract; a single contract may have multiple receipts over time if it is a pawn contract (buy and sell contracts will have a single receipt).

Even if a new receipt is not issued when a pawn contract is cancelled, there'll need to be a database update of the contract or the receipt (or both) to indicate that the contract is cancelled.

Is there anything that I'm missing here?

I'm not sure why you need different tables for different product details. I'm going to guess though, that it's because your product types are so different that they have very different attributes (clothing will have sizes: S, M, L whereas books will have have an indicator of hardcover vs. paperback, etc...). You might want to try a more generic solution, where you have a table of product attribute types and another table of attributes. Something like this:

product
-------
  product_id
  name

product_attribute_type
----------------------
  attribute_type_id
  attribute_type_name

product_attribute
-----------------
  product_attribute_id
  attribute_type_id (FK to product_attribute_type)
  product_id (FK to product)
  value

Then you can have data that looks like this

product
-------
ID | name
-----------
1  | Awesome pants
2  | Book of cool stuff
3  | 1337 software

product_attribute_type
----------------------
ID | attribute_type_name
------------------------
1  | waist size
2  | number of pages
3  | system requirements
4  | inseam

product_attribute
-----------------
attribute_type_id | product_id | value
--------------------------------------
1                 | 1          | 33 inches
4                 | 1          | 34 inches
3                 | 3          | Win7, 4 GB RAM
2                 | 2          | 703 pages

Getting info on a product is easy:

select product.name, product_attribute.value, product_attribute_type.name
from product
inner join product_attribute on product.product_id = product_attribute.product_id
inner join product_attribute_type on product_attribute_type.attribute_type_id = product_attribute.attribute_type_id
where product.product_id = $PRODUCT_ID;