Those screws held the bracket...
With coolant go 50/50 distilled water with factory concentrate or this stuff:
https://www.valvoline.com/en-australia/our-products/coolant-and-antifreeze/zerex-g-05-antifreeze-concentrate
The Aussie cars had a metal bracket that held a yellow halon (BCF) extinguisher. I prefer these but they’re not kind to the ozone layer apparently:
http://www.environment.gov.au/protection/ozone/halon/halon-disposal
Similar to Dkubus, these guys can supply all sorts of performance parts for early Mercs. Apparently they’ve been around for years but I only just found them on FB... Another well kept secret unearthed!
http://elbe.ee/contact/