Loaded up a ASR903 with:

#dir stby-bootflash:
Directory of stby-bootflash:/

[other files]
23 -rw- 493827100 May 8 2019 0431 +00:00 asr900rsp3-universalk9_npe.03.18.01i.SP.156-2.SP1i-ext.bin
24 -rw- 327771140 May 8 2019 0529 +00:00 asr900rsp2-universalk9_npe.03.18.03.SP.156-2.SP3-ext.bin

But a colleague tells me the running version doesn't match either of these IOS files:
#show ver
Cisco IOS XE Software, Version 03.17.01.S - Standard Support Release
Cisco IOS Software, ASR900 Software (PPC_LINUX_IOSD-UNIVERSALK9_NPE-M), Version 15.6(1)S1, RELEASE SOFTWARE (fc3)

We both agree the boot-start-marker definitions need a bit of tidying up, but not sure how to do that:
boot system bootflash:Image/packages.conf
boot system flash asr900rsp3-universalk9_npe.03.18.01i.SP.156-2.SP1i-ext.bin
boot system flash asr900rsp3-universalk9_npe.03.18.03.SP.156-2.SP3-ext.bin
boot system flash asr900rsp2-universalk9_npe.03.18.03.SP.156-2.SP3-ext.bin

I made a typo entering one of the IOS image names (the middle file), and now we can't get rid of it - but I'm not 100% convinced that is the actual problem. Why won't the box apparently load asr900rsp3-universalk9_npe.03.18.01i.SP.156-2.SP1i-ext.bin? It's MD5 calculates out OK, and is present on both main and standby bootflash.