contract/analytic_hours_block/project_view.xml

<?xml version="1.0" ?>
<openerp>
    <data>

        <record model="ir.ui.view" id="edit_project_hours_block_link">
            <field name="name">project.project.form.hours.block.link</field>
            <field name="model">project.project</field>
            <field name="inherit_id" ref="project.edit_project"/>
            <field name="type">form</field>
            <field name="arch" type="xml">
                <xpath expr="//button[@name='attachment_tree_view']" position="after">
                        <button name="hours_block_tree_view" string="Hours Block" type="object"/>
                </xpath>
            </field>
        </record>


    </data>
</openerp>