I beg to differ - you're leading Fairwheeler astray.
There are many different configurations of shifter internals in LH levers, and although the basic principles of return and detent springs and pawls are found in all Shimano shifters, how they're laid out in a particular model is very variable between them, over the 30 years that they've been made. They're not, apart from a couple of models of mid-90s underbar shifters, similar right to left, as a rule.
The only way to repair a lever, unless you have a comprehensive library of printed exploded diagrams (the digital ones disappear from techdocs after a few years), is to draw diagrams as you disassemble, or take photos, lay the parts out in trays or carefully on your clean and tidy workbench, and work from outside in. Pretensioning springs on reassembly and getting the attachment plates orientated correctly is the tricky bit. Sometimes it's possible to hook a spring back into place if it's popped out, if you can get a dental pick into the hooky end, you might be able to reseat it, but it's popped out for a reason, and is probably stretched or bent as a cause, and will do so again. Rebending springs to their former shape has only an average chance of success. Try your luck, it can't hurt.
If the spring has snapped, it's all a bit moot anyway. If you've got a broken component inside, unless you have another lever to "cut-and-shut" with (as panel beaters say) or cannibalize, you're whistling Dixie. There's not many (if any) available replaceable parts on 7 speed levers - they cost ~$35 a pair complete nowadays (I know that's not the point to mechanics, but it is to Shimano).
So, cost effectiveness for the operation is low (you might be swearing at it for several hours, especially the first few times, with low probability of success), Frustration potential high. Some older 8 and 9 speed levers are serviceable, but rarely repairable.