My first M60 was actually a Glenfield 99G (Glenfield was the
low price brand name for Marlin guns) and it had takedown
screws where the modern versions have the nylon split pin
and the little posts up front.
Comparing the old to the new: the new has a bolt-lock
feature with the bolt handle, the ejector is an extension of
the cartridge lifter spring, the new has a last-shot bolt hold open,
and there is more room in the receiver for gunk to accumulate
before you have to clean. The current firing pin is lighter
than the original.
On the sideplates that contain the action parts: on the new,
the pins are permanently mounted to one of the action plates,
while the other action plate was held on by C clips over the
protuding ends of the pins. On my Glenfield 99G, there were
C clips on both ends of the pins.
On the older models, the ejector was part of the feed throat,
which is a hard chrome plated cast aluminum piece. If the
chrome wears through (after, say, 20,000 rounds) the ejector
soon becomes useless. The new ejector is much more robust.
I have retro fitted an old Marlin with a new style ejector and
the current owner is quite pleased with the result.
Eventually, you can wear out the feed throat itself, allowing
cartridges to ride too high in the feedway. I have replaced
two of feedthroats for this reason and it is not a fun thing:
the action parts between the two sideplates should only be
disassembled for replacement of parts if needed and for
routine cleaning, well, thats what tooth brushes, Q-tips, and
pipe cleaners are for: to clean the firing mechanism of the
Marlin Model 60 without breaking down the parts within the
left and right sideplates.
Eleven million of these babies have been sold!
The Model 99 I believe actually came out in 1959. There was
also a version called the 49 that had a two piece stock.
And the 99M1 with a M1 carbine style stock with handguard
and short tube magazine and the 99M2 with clip feed.
(Yes, Marlin called the magazine a clip: go after Marlin for that.)
Since then the tube magazine versions have usually been called
Model 60 and the detachable box nagazine versions Model 70.
The Model 70 ejector is part of the magazine, or, clip.
The original versions had 22 inch barrels and full length tubes
for 18 round capacity. Then the New Jersey assault weapon ban,
and Marlin shortened the magazine to 14 rounds. Today, they
have shortened the barrel to match the magazine length.
For export Marlin did make a Model 60 with a ten shot tube for
countries with even more drastic magazine limitations.