Difference in location of transaction isolation level declaration - inside/ouside a transaction?
Below are relevant excerpts from the SET TRANSACTION ISOLATION LEVEL documentation.


  Only one of the isolation level options can be set at a time, and it remains set for that connection until it is explicitly changed. All read operations performed within the transaction operate under the rules for the specified isolation level unless a table hint in the FROM clause of a statement specifies different locking or versioning behavior for a table.


It goes on to say:


  If you issue SET TRANSACTION ISOLATION LEVEL in a stored procedure or trigger, when the object returns control the isolation level is reset to the level in effect when the object was invoked. For example, if you set REPEATABLE READ in a batch, and the batch then calls a stored procedure that sets the isolation level to SERIALIZABLE, the isolation level setting reverts to REPEATABLE READ when the stored procedure returns control to the batch.


Note that there is no mention of BEGIN TRANSACTION, COMMIT, etc. These transaction control statements do not affect the current session transaction isolation level.

I don't see a functional difference with specifying the transaction isolation level immediately after the BEGIN TRAN. Perhaps the developer wanted to make sure the level was obvious to the reader and thought the best place was at the transaction boundary rather than having to view the beginning of the proc code.