It sounds rather "old fashioned" to me, to have a card in order to be able to participate in plays/shows?
I'm all for representation to protect your rights in any field and should never be thought of as old fashioned. In essence the card was your membership of the Equity union who look after member's rights. I've no doubt in the early days of the profession, some (particularly women) were treated unfairly, poorly paid, owed money, bullied, dismissed for no reason and the like, so it was an advantage to be represented officially.
On the other hand, as was mentioned earlier, I think you had to join Equity, but at least there was a platform to air your grievances if you felt you'd been treated unfairly.
On the topic of cast and ages, I didn't realise Arthur was only five years older than Clive. It doesn't come across in the episodes where Clive springs around like a youngster while Arthur ambles along at a slower pace - and yet Clive's character is meant to be much older than Mainwaring. In terms of characters, shouldn't the Captain be more mobile than Jones?
Clive was an acrobat, a unicyclist, a dancer and did his own stunts, etc. So he was certainly agile and nimble.
Not though, in the context of the character. He was an old veteran, likely well past retirement age, whereas Mainwaring was supposedly a good few years younger. The difference seems lop-sided with the older guy full of energy and the younger man unfit and slow. It was just an observation. In real life there was five years difference, but in the show it was anything between ten and fifteen. We'll put it down to the Captain carrying a few pounds extra!