This is SEVA spacer, usually placed between SanDI and SwaI restriction sites.

The Standard European Vector Architecture (SEVA) platform is a digital and physical repository for de-constructing and re-constructing complex prokaryotic phenotypes. SEVA provides a uniform syntax and vector architecture composed of preformatted arrangement of functional DNA modules: replication origins, antibiotic resistances and cargo modules. All plasmid modules were exchangeable and suitable for any Gram-negative host. SEVA is promoted and maintained by the Spanish National Centre for Biotechnology (CNB) in Spain. For more information, please visit SEVA home.