I have a Model 61 serial number869950 that shoots shorts, longs and long rifle. This serial number and this combination of 3 different ammos seems unusual as I cannot find any info on this gun. Is this really a model 61 or possibly something else? Thanks

This can only be answered by a physical examination of the weapon or high resolution images of the whole weapon

this is copied from a site, the web address is pasted below.

The serial number depending on the model of rifle, will be located either on the upper or lower stock tang, or side of the receiver.
http://www.gunvaluesboard.com/marlin-serial-numbers-dates-of-manufacture-1664.html

also if you find serial # you may find more info here.
http://oldguns.net/sn_php/marlinlookup.php

From 1962 thru 1968 Marlins rimfire rifles (other than the 39 series lever actions) used a two letter date code that was stamped on the barrel. This date code is different than the one already listed, as Marlin rimfire rifles (other than the model 39 series) did not have a serial numbers until required to by Law after 1968. The first letter of the date code represented the month of manufacture while the second letter indicated the year. This date code was usually stamped on the left side of the barrel just in front of the receiver.

It's a Model of 1873 'Trapdoor'. LOW POWERED ammo ONLY. Do NOT shoot ammo designed for Marlin or Ruger.

It was manufactured in 1906.

.32 rimfires were discontinued after America entered WWII.

You MIGHT still find some old stock somewhere, but I would feel they are questionable from being that old.

.32 rimfire were made primarily in short and long lengths, but extra short, long rifle and extra long lengths were also made.

Check which part is hotter than others, see if applying of a lubricant help in solving the jamming problem of your rifle.

yes,
Most any good gun dealer should have some, I prefer the target model with the heavy barrel. Adjustable sights ar a must for any accurate shooting with a pistol, also use only the standard velocity ammo or the target ammo. you will be far more accurate with these.

it dont really matter what they are,,if you use one make of ammo it will not shoot the same when you use another make of ammo,,,the best thing to do is test fire the rifle and work it out? with the type of ammo you will use all year long?

You might have to try another brand of ammo. My Mossberg only shoots straight with Winchester ammo.