Here's a question for all of you 99 PhD's, how effective were those windage adjustment wheels on that Williams scope mount on the OP's 99R post. Often wondered about those. Saw a 99 at a Des Moines (Ia) gun show not long ago that had one on it. Asked the seller and he had no idea.
Those left side receiver screws (and holes if the screws were out) remind me of alot of Rem 8/81 rifles I've seen over the years. My first 81 (late 40's) I got didn't even have plug screws in it. Bought some from Brownells that were tapered heads that had sides that fit flush with the receiver. While they looked out of place on the rifle IMO, they looked better than just plugs or normal flat head style. Have seen pictures of 8/81 left side receivers that had more holes in them than the proverbial 'Black Holes of Calcutta'! Past owner/owners must have tried every mount available at some time. Nasty lookin rifles. Holes in 8 and 81's get the same devalue ($$) as most 99's that do so and/or have top receiver D&T of pre-factory tapped 99's.
Rifles with no mounts on the Rem 8/81 forum with numerous plugs/screws in them get the same "Wonder what scope mount those are" questions from time to time. The receivers on those rifles are sort of thin also, maybe not as bad as a 99, but after D&T, there is limited steel available. Screw shanks inside of the receiver need to be flush also with the inside so as not to interfere with the bolt moving back and forth. I did get a Williams style side mount that the holes were for not long ago but haven't mounted it yet. Have a vintage Weaver scope from the 50's to mount on it. Thankful for the Stith and Lightfoot 99 scope mounts. While the Stith is vintage, Mike's (soon to be Gary's) is more versatile than the Stiths for scope selection.