[ hap]
The DMLBS notes the use of the p.ppl. of palmare as sb.f. meaning ‘slap of blow given with the flat of the hand’. The sense of ‘span’ is one of the uses of palma (DMBLS 2088a). If this is their etymology, it may be that the forms in the citations under ‘span’ should be read as having unaccented final vowels.