I've had a look at the photos, that there is of them.
I've compared the markings on the one shown in the original post with the markings of my SPF 6067 Rifle (7.62mm) Assembled in 1963 and looked at a number of other SPF rifles markings and ALL are 7.62mm rifles. The SPF number is a census number, they same why New Zealand originally applied the N/|\Z marking. Census numbers tend to be applied by the crate or rack and have nothing to do with the factory serial number for example if you look at the list below, the dates of manufacture are all over the place in comparison to the SPF number. Armourers have enough to do, without the headache of applying a census number, hence the general rough application of the hand stamped numbers........ sounds like a job for the apprentices.
SPF 473....... AD70 (7.62mm)
SPF 4111..... AD67 (7.62mm)
SPF 6067..... AD63 (7.62mm)
SPF 6093..... AD70 (7.62mm)
SPF 7264..... AD68 (7.62mm)
SPF 7675..... AD70 (7.62mm)
SPF 7751..... AD67 (7.62mm)
SPF 7784..... AD69 (7.62mm)
I've never come across an
Australian Lithgow made .22 L12A1 Rifle, They didn't make any such thing.
if you look at the markings you can clearly see how Lithgow roll stamp their markings, they DON'T use Serif's on their '1', its just a 'l'. The 'F' doesn't match either.
As Peter pointed out there are instructions EMER/EMEI Instructions dealing with what to do with a rifle with a stuck L12A1 barrel. Its possible a workshop in Singapore altered some rifles for the SPF, but I would expect to see some form of marking from the workshops. The L12A1 didn't come arrive till about 1966/67, I can't see them being used by the SPF for a decade, a bit like New Zealand and Australia. Realistically they didn't see the kits until the late 70's and were restricted use. I can't see the SPF getting them earlier.
https://www.milsurps.com/images/impo.../yvMiqhC-1.jpg