As others have already said, each item of stores/equipment has its own L number series, so an L number for one type of equipment has no connection with the same l number for a different type of equipment.
In your case, the 7.62mm L42A1 round has nothing to do with the L42A1 sniping rifle, so don't worry about that.
The round you have is the first of a series of sniper/match cartridges with a 155 grain bullet (instead of the normal 144 grain ball bullet). There are also L42A2 and L42A3 rounds, the only difference being the supplier of the propellant powder.
The 7.62mm L numbers have now progressed to the L57A1 which is a ball round made by MEN in Germany.
It gets even more confusing. Hangerman wrote that "..the first 7.62mm ball round was the L1A1" which is not actually the case. The first L number allocated to the 7.62mm series of cartridges was the L1A1, the first ball round was the L2A1. Now for the confusing bit! In 7.62mm calibre there are two L1A1 designations, one a drill round and the other a grenade discharger, and it has often been said that this is an error in allocating L numbers.
In fact it is not an error, as the Drill round is "Round, 7.62mm, Drill, L1A1" because it has a bullet, whilst the grenade cartridge is "Cartridge, 7.62mm, Grenade, L1A1" because it does not have a bullet. Originally "Cartridges" and "Rounds" had their own independent L number series but this seems to have been dropped over recent years since the later blank rounds like the L31A1 (which are technically cartridges, having no bullet) are numbered in the same series as ball and tracer rounds.
I hope that helps, as I realise how confusing the nomenclature is to someone just starting out, but there is a method in their madness!
Regards
TonyE