رِجَالٌ: broken plural, normally fully declinable; genitive رِجَالٍ.
كُتُبٌ: broken plural, normally fully declinable; genitive كُتُبٍ.
مَسَاجِدُ: on the book's specified pattern; bare indefinite genitive مَسَاجِدَ.
Being a plural and belonging to ṣīghat muntahā al-jumūʿ are not the same thing.
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Diptote Causes — Ultimate Broken-Plural Patterns
رِجَالٌ: broken plural, normally fully declinable; genitive رِجَالٍ.
كُتُبٌ: broken plural, normally fully declinable; genitive كُتُبٍ.
مَسَاجِدُ: on the book's specified pattern; bare indefinite genitive مَسَاجِدَ.
Being a plural and belonging to ṣīghat muntahā al-jumūʿ are not the same thing.