Good evening,
Firstly I am not a collector of Penny blacks, but I have a friend who is and whilst looking into something else came across a resource that might be useful to others if they do not already know of it.
It is a specific page of the 'Stamps of the World' Wiki site which is organised by letters such that if you have a penny black with 'PB' letters, you click on the "Penny Black PB" link and it takes you to a page showing images of all the plates for the PB position on the sheet. If you click on the plate image a larger, better quality image then opens allowing anyone/everyone to have a go at identifying the plate.
Apologies if everyone knew of this, and I cannot vouch for the quality/reliability of the data - but I thought it was well worth putting out there in case it was not well known the link is below:
https://www.stampsoftheworld.co.uk/wiki ... FqU73ceLH0Have a great evening,
H