You need to determine what model Stevens Tip-up pistol you mean - as there were several models made (#35, #37, #40, #41, etc, etc).
Most used a similar mainspring, as the models were simply stylistic variations of the same pistol - but a few were different.
Some pics of the pistol (including it's sights), and particulars about barrel length, would help.
Here's the most common mainspring type:
http://www.e-gunparts.com/product.as...SKU=917950&MC=
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