I wouldn’t have noticed the spelling
Shouldn’t it also have just one “l” - spoonsful.
Searching Google books for “teaspoonfuls”, it comes up with several books etc from the 19th century that include that spelling. For example, it is in Mrs. Beeton's Dictionary of Every-day Cookery from 1865:
https://www.google.co.uk/books/edition/Mrs_Beeton_s_Dictionary_of_Every_day_Coo/hxQqAAAAYAAJ?hl=en&gbpv=1&dq=Teaspoonful+dictionary&printsec=frontcoverTeaspoonfulls also has some results from a similar period, and also spoonfulls without the tea, I think maybe they just couldn’t spell or didn’t care.
It looks like the numbers have been wheel engraved and the words diamond etched which is curious too, I think normally they just wheel engrave the whole thing resulting in a blocky looking font. The diamond etching looks quite classy and nicely done, no idea when though.
It might help to measure the volume of their spoons as it looks like the unit size changed at some point (teaspoons on wiki). It could also be for apothecary use.