Greetings mowers, and welcome to the ODK Forums.

It looks like you have a SB Model 16 'The Queen', 24" cut version. These were first made in 1950, so it would most likely be 1950's or 1960's vintage, maybe as late as 1970's.

Some history and a manual are here: https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/75539/

And this model is featured in a 1963 SB catalogue HERE.

As you surmise, the Robin engine would not be original - this machine most likely had a Villiers. However, this model is very much built to 'pro' quality and durability, so outlasting its original engine is no surprise.

Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."