It is very hard to tell a 23/ 1 body from a No 5. There are characteristics of both in each mark which can be seen when the grens are found. Later gren bodies have the pinholes drilled further into the lugs and the pinhole is rebated, i.e, the pinhole is slightly recessed but i have seen dug no 5s with this and 23/1s without it. Earlier produced bodies would just not be wasted.
Also, with the numbers that were produced, 65 Million, during WW1, large numbers of no 5 bodies were used as 23/1s just with the addition of the 23 base plug due to such a vast number of them being produced.
I would say its very difficult to be precise here. I have a really good 23/2[one of my best Mills] with a plug dated 4/17 and if you put a no 5 plug on it, i could not tell if it was wrong, and i can assure you i have scrutinised it for this reason. I have no doubt over its originality.
However, a general concensus is that if the pinholes are deep, ie, further into the lugs its a 23 but i dont think this is a defining characteristic.
Andy