Arty,
the mower with the tag 2048, indicates this is a 20 inch cut built in 1948 (English mower), re the mower with no tag, remove the aluminium cover off the engine flywheel (held on by 3 screws) stamped on the flywheel you will find numbers stamped, I have found on the ones I have looked at 3 sets of numbers, reading them from the left the last one on the right is the date.
To confirm this take the cover off the mower with the tag as well it should have 48 stamped there, this is only an indicator as the mower with no tag could have had an engine transplant over its lifetime.
Have fun, I started on one of my 5 Atcos yesterday very simple mower, providing each mower is complete, not all mine are so one may end up as a spare parts mower.
Beech.
