▁about ▁this ▁can ▁be ▁determined ▁by ▁a ▁three - number ▁code ▁on ▁the ▁post marks . ▁ ▁In ▁Great ▁Britain ▁the ▁first ▁post mark ▁employed ▁for ▁the ▁can cellation ▁of ▁the ▁then ▁new ▁ad hes ive ▁post age ▁st amps ▁was ▁the ▁M alt ese ▁Cross , ▁so ▁named ▁because ▁of ▁its ▁shape ▁and ▁appearance . ▁This ▁was ▁used ▁in ▁conj unction ▁with ▁a ▁date ▁stamp ▁which ▁was ▁applied , ▁usually ▁to ▁the ▁rear ▁of ▁the ▁letter , ▁which ▁denoted ▁the ▁date ▁of ▁posting . ▁ ▁D ifferent ▁types ▁of ▁post marks ▁include ▁railway ▁post ▁offices ▁(" R PO s ") ▁and ▁mar itime ▁( on - board ▁ship ) ▁post marks . ▁ ▁Post marks ▁on ▁naval ▁vessels ▁during ▁sensitive ▁operations ▁in ▁w art ime ▁are ▁sometimes ▁" clean ", ▁showing ▁less ▁information ▁than ▁normally ▁to ▁prevent ▁route ▁of ▁travel ▁or ▁other ▁details ▁from ▁falling ▁into ▁enemy ▁hands . ▁ ▁Similar ▁to ▁this ▁is ▁the ▁" c ens ored ▁post mark ," ▁over print ed ▁with ▁a ▁black ▁ob liter ation ▁of ▁the ▁time ▁and ▁place ▁of ▁ma iling ▁for ▁similar ▁reasons . ▁ ▁The ▁P ony ▁Express ▁used ▁a ▁variety ▁of ▁different ▁post marks ▁on ▁the ▁mail ▁it ▁carried ▁across ▁the ▁Western ▁United ▁States . ▁There ▁are ▁only ▁ 2 5 0 ▁known ▁examples ▁of ▁surv iving ▁P ony ▁Express ▁mail / post marks ▁in ▁various ▁collections ▁today ▁bearing ▁one ▁of ▁more ▁than ▁a ▁dozen ▁different ▁types ▁of ▁post marks . ▁ ▁Hawai ' i ▁post ▁once ▁had ▁a ▁sur f