The Met Office has 1km grid rainfall data going back to 1891 but with local thunderstorms - particularly prior to 1960 - it is often the case that they slip through the net if there is no reporting station nearby and the storm is very localised
In this case, the details are captured very well on the 1km grid for that date and location as shown below
There was heavy rainfall of 25-50 mm over a ide area but the centre of the storm seems to be about 10 miles SW of Louth and recorded about 120mm rain
As with Lynmouth, Boscastle and many others, the local river system rapidly overflowed and transported the floods downstream in this case to Louth as marked