The 99M1 Marlin differs from the 99 in that it was indeed made to pay homage to the Garand/Carbine weapons of the M1 designation. They were fitted with a shorter magazine tube (9 rounds instead of 15) to make it less noticeable, and also are equipped with sling swivel anchors, a barrel band, and an over-barrel wood handguard. The rear sight is a removeable, adjustable unit with more of a GI-look than the ones used on the 99.
I have two of them. One is a pre-'69 (no serial number) and that one is intact, complete with it's rear sight (these can fetch $80 or more, as many owners removed and subsequently lost them when they scoped their guns.) This gun appears in original and unrestored condition.
The other one has been nicely refinished, and has a sling and scope on it. The original rear sight is missing. It has a serial number, and was probably made around 1970.
I got the first one maybe three years ago for $140 at a LGS, and the second one maybe one year ago for about the same.
Admittedly, I have not yet gotten around to shooting them. I'm looking forward to getting a chance in the coming month or so; I was just going through a phase during which I was acquiring rimfire guns faster than I was ammunition for them. But, these guns feel solid and sturdy in the hand, and I expect they'd help satisfy a thirst for a M1-styled .22 rifle. Now, if I could only come across a 99M2; these used a detachable box magazine instead of the tubular unit..