Switching

experts,

I have noticed on 2200C:

Boot Media: internal (da0)
Active Partition: da0s2a
Backup Partition: da0s1a
Currently booted from: active (da0s2a)

and 

Information for snapshot on internal (/dev/da0s1a) (backup)
Creation date: Jun 20 20:51:58 2018
JUNOS version on snapshot:
jbase : ex-12.3R12-S7
jkernel-ex-2200: 12.3R12-S7
jcrypto-ex: 12.3R12-S7
jdocs-ex: 12.3R12-S7
jswitch-ex: 12.3R12-S7
jpfe-ex22x: 12.3R12-S7
jroute-ex: 12.3R12-S7
jweb-ex: 12.3R12-S7
fips-mode-arm: 12.3R12-S7
Information for snapshot on internal (/dev/da0s2a) (primary)
Creation date: Nov 24 06:39:35 2017
JUNOS version on snapshot:
jbase : ex-12.3R12-S7
jkernel-ex-2200: 12.3R12-S7
jcrypto-ex: 12.3R12-S7
jdocs-ex: 12.3R12-S7
jswitch-ex: 12.3R12-S7
jpfe-ex22x: 12.3R12-S7
jroute-ex: 12.3R12-S7
jweb-ex: 12.3R12-S7
fips-mode-arm: 12.3R12-S7

other 2200C shows the main partition is 

Other 2200C showing  da0s1a instead of da0s2a - is this just normal and I can leave it as it is ?

It doesn't matter if da0s1a or da0s2a is the primary partition. Primary and backup changes roles without any operational difference.

Next time you upgrade it will upgrade the backup partition, reboot and make it the primary partition (if the upgrade goes as planed). Same with an unplanned shutdown where da0s2a "breaks", it will just boot on da0s1a.

If you really wan da0s1a to be primary you can reboot on the backup partition with the command 'request system reboot slice alternate'

Thank You Jonas for a quick reply and explanation !!!  have a nice day