What differs between فَلْسٌ and فَرَسٌ?

Noun Patterns: Jāmid, Maṣdar, Mushtaq and 50 Basic-Triliteral Maṣdar Entries

• Their vowel/sukūn template
• The first has four radicals
• The second is a mīm-form maṣdar
• The first has a prefixed hamzah