This link has an article starting on page 52 about MET recce flights in WWII. On page 55 is mention of a method of computing altitude at night based on timing the dropping a "can" which would flash, apparently when it hit the water. That may be the "flash"
altimeter. There was a table that translated the times into altitudes. Once they had a estimate of altitude based on the flash, they could reset the aneroid altimeter to that altitude.
https://www.rafmuseum.org.uk/documen...Operations.pdf