Need the peep part of a peep site for a marlin model 80D 22 cal. rifle.

1964

https://www.gunvaluesboard.com/marlin-serial-numbers-dates-of-manufacture-1664.html

https://www.google.com/search?q=Marlin+Limited+Edition+model+6081Year+of+manufacture

..

Hope this what your looking for:
https://www.gunpartscorp.com/gun-manufacturer/marlinglenfield/rifles-marlin/20a

Although you didn't specify the model, it sounds like you have a Marlin model 60 of which over 11 million have been manufactured from 1960 to present. It looks like yours was made in 1975. Most of the gun auction sites are listing that model for $60-125 on average.
Marlin Model 60

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.

If it's new in the box, around $135 to $150.

TRY THIS LINK
http://www.gunvaluesboard.com/marlin-serial-numbers-dates-of-manufacture-1664.html

1966