Ian,
I've been doing a lot of 1930's forest service aerial photos and a lot of them do not have fiducial marks and have been working on them with PhotoScan/Metashape the last 3 years . At first it was hard for me because I have never experience with these Photogrammetry skills ( I am GIS Specialist) and work with my co-worker ( He is going to retire at this end of year) and we work together the last few years and made some suggest to me on some of the areas I have learned.
It is learning curve for me but I always work on and make efforts to make it work. A lot of my photos are mostly on the mountains ( I am in Colorado ) and in some areas are flat areas.... What my co worker found that a few I have worked on them and he found a lot of my work excellent . He was very satisfied with my work... The hardest part is finding the feature on the old photos vs the today's photos.
It can be very frustrate at first but the more you work the better understand then you will be very good at it. I have learned a lot over the years now.
So without the fiducial marks, it still can work unless if there is some bad aerial photos I have faced with it...
So maybe if you could show us your original scanned aerial photos you have
Here is an example of my work of the same area Before and After.
The Before Orthos had some black markers is because of the aerial photos I have and the After Othos, I did clean it up and the black markers disappears.