There were some definitive differeances between the BP and Nitro for BP cartridges (Low pressure) with those for “Nitro” only. (Higher Pressures).
One differance was that most “Nitro” cases were slightly longer than those for BP or equivalent ( 3inch BP, 3,25inch Nitro.)
Also, some makers made proprietary cases with thicker or thinner rims.
The Bullet type is also instructive, as Lead and BP was subjected to fouling, so rifling design was generally different from Rifles for Jacketed Bullets ( not always).
BP and Nitro for BP were loaded to the same Proof ( pressure) levels, so that the cartridge could interchange in the older guns; Purely Nitro guns, as mentioned, had different cases, and possibly to prevent “accidental interchange” and a disaster.
Whilst a 3 inch BP or Nitro for BP could chamber in a 3,25 Nitro chamber, no harm would be done ( except for the fouling up of the rifle), but if one managed to squeeze a Cordite loaded Nitro express into an early BP rifle, all Hades could break loose.
( as would the rifle).
One must go by the headstamp, a set of calipers, and a good book on English Hunting cartridges, to finally differentiate between all the different types.