Methods of manufacturing a plurality of mats useful with a plurality of frame style seal members in a load-supporting surface include at least one milling machine, router or pre-formed mold forming at least first and second elongated grooves in first and second offset outer edges of at least a first side of each among at least first and second stepped-configuration, reversible mats. Each groove causes the seal member(s) seated therein to extend into a gap formed between the mats, abut at least one seal member extending from another mat into the same gap and form a liquid-tight seal therewith.