Filter for Takumar 500mm M42 lens

Hi all...just recently bought a secondhand Takumar Telephoto lens
500mm, M42mm screw mount. I want to screw a filter to the front to protect the lens and want to know the size...I know the rear of the lens took a 49mm sized filter. I can't find any info on what size filter would fit on the front...

Thanks in advance

If you have the lens cap, the size should be on the back of it.

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Pentax K5, MZ6
Pentax Lenses 28-80 F, 18-55 WR DA, 50-200 WR DA, 300 DA*, 80-200 F, M 20, 35 F2.4 AL, M50 F1.7
ONE UNITED Member

No it wouldn't, these lenses have a cast and machined metal filter that pushes on over the lens barrel.

TBH It would be difficult to imagine a situation where you would be so close to anything as to collide with it and require any such dubious 'protection'.

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Ken