Spanner for SU HS Carbs

15mm square steel tubing, with a hex formed around a correctly-sized bolt head - 0.6" or 15.24mm, found by using a 5/16 Whitworth, 3/8 BSF or 19/32 AF spanner or socket. One flat cut off the fit over the enrichment lever and jet tube, and below that cut back another half-flat to allow the spanner to turn the nut a flat at a time:

The jet adjusting nuts on HS carbs: