Birkenstock Flen Mocha Brown Suede Leather Lace-Up Ankle Chukka Boot Shoe. Size 13-13.5 US, 46 DE. Excellent, gently used condition. Light normal wear and some water spots (please see photos for details). Elevate your shoe collection with these men's Birkenstock chukka boots. Crafted with comfort in mind, these boots feature a suede upper material and lace-up closure for a secure fit. The brown color and solid pattern make these boots versatile for any occasion.