I believe you could spot a serious player as they would grow the fingernail
on their forefinger!
Indeed. I played for England at Under 16 level in the mid 90s - growing fingernails was fairly common in tournament Subbuteo, or for a few rogue types using false nails (which was frowned upon, but not against the rules).
Base modification was common from (at least) the early 80s onwards. The common things were ...
- Pop the middle disc out and use plastercine to add extra weight (total weight between 2.0g and 3.2g depending on preference for ball control to power)
- Use sandpaper/glasspaper to lower the base profile & increase the size of the flat on the bottom (to ~15mm), finishing off with t-cut
- Paint the bottom of the base with a 50:50 mix of water-based varnish and ethanol, then polish the base before every game with furniture polish
The extra weight & lower profile makes shooting / chipping the ball much easier. The wider flat and varnish mix means they're more stable, and can be (accurately) flicked down the entire length of the pitch.
In competitions, formations tend to be 6-4-0 (rare), 7-3-0 (common), or 8-2-0 (rare but gaining popularity). It's worth bearing in mind that figures naturally drift up the pitch, and are rubbish at tracking back, so they're more like the formations you'd seen in real football when the team's defending. In actual use, they're closer to 3-3-4, 4-3-3, and 5-3-2 respectively. 7-3-0 is traditional, and fairly balanced. 8-2-0 puts more emphasis on heavy counter attack, and is reliant on long distance flicking accuracy. 6-4-0 puts you in for an intensive, high pressing game - it needs a load of discipline (or a much weaker opponent) to pull off.
Edit: there's also hyper-rare formations that involve actual strikers, something like 7-1-2 or 6-2-2. The problem is that out-and-out strikers only really work if you're playing long balls, which is a risky tactic. It
can work well with heavily weighted figures, but it's a bit of a leftfield tactic that's the Subbuteo equivalent of sending a keeper up for a corner.