How can I make this query sargable?

I have a query which joins two tables based on a Timestamp value (don't ask).

One of these tables stores the data in a column with a Timestamp data type. The other stores it as a varchar.

Previously, the query converted the timestamp column to a varchar for the join. As I understand it this is not sargable, so I'd like to do the reverse. (The "Timestamp" table is much larger and efficiency is more of an issue).

SELECT * FROM TABLE1 INNER JOIN TABLE2 ON UPPER(master.dbo.fn_sqlvarbasetostr(cast(TimestampColumn as binary(8)))) = VARCHARTIMESTAMPCOLUMN

I've tried:

CONVERT(ROWVERSION, N'0x0000000003306BDD')

No error, but no match on the table (when a row definitely exists).

CONVERT(ROWVERSION, CAST(N'0x0000000003306BDD' as bigint))

This gives an error converting nvarchar to bigint.

How can I convert the varchar value into a timestamp rather than the other way around?