Just guessing here but I would hazard a guess that if the platform was installed back in the '70s when it was installed, a welder and his mate looked at what was needed and knocked something up, (Which was more than likely suitable and they did a good job).

My personal starting point today would be to get an engineer to look at it even if it was constructed to an Engineered Design,  who would probably suggest an NDT welding inspection, (MT or UT), that can be done in-situ, who should then be able to give you a SWL/WLL recommendation, and then you've got a 'baseline' starting point you can work from and get it Made Safe if required.

To quote the well worn phrase, IMO, the checking would then be on a Risk Based Assessment, dependant on amount of use taking into account the means of access/egress, guardrails, (and any W@H inspection schedules for Anchor points for harnesses) etc if applicable

Or, if you have them onsite, dismantle the platform and use a MEWP/Cherry Picker...... or PASMA tower depending on the height of course.

Edited by user 02 January 2024 09:59:56(UTC)  | Reason: Not specified