Fail to override interface module in magento2

i need problem with extension interface in magento2.

Mi extension_attributes.xml:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="MagentoSalesRuleApiDataCouponGenerationSpecInterface">
        <attribute code="specific" type="string" />
    </extension_attributes>
</config>

Mi di.xml

<preference for="MagentoSalesRuleBlockAdminhtmlPromoQuoteEditTabCouponsForm"
            type="CustomSalesRuleBlockAdminhtmlPromoQuoteEditTabCouponsForm" />
<preference for="MagentoSalesRuleHelperCoupon"
            type="CustomSalesRuleHelperCoupon"/>
<preference for="MagentoSalesRuleModelServiceCouponManagementService"
            type="CustomSalesRuleModelServiceCouponManagementService"/>
<preference for="MagentoSalesRuleModelCouponMassgenerator"
            type="CustomSalesRuleModelCouponMassgenerator"/>
<preference for="MagentoSalesRuleModelDataCouponGenerationSpec"
            type="CustomSalesRuleModelDataCouponGenerationSpec"/>

Mi model/data:

<?php
namespace CustomSalesRuleModelData;

class CouponGenerationSpec extends MagentoSalesRuleModelDataCouponGenerationSpec
{
    const KEY_SPECIFIC = 'specific';

    public function getSpecific()
    {
        return $this->_get(self::KEY_SPECIFIC);
    }


    public function setSpecific($specific)
    {
        return $this->setData(self::KEY_SPECIFIC, $specific);
    }
}

The $specific return null ever. My CouponGenerationSpecExtensionInterface.php and CouponGenerationSpecExtension.php extended setSpecific y getSpecific but i never can enter in $specific.